It has been shown that there is a decrease in the content and activity of three neurospecific proteins (neurospecific enolase--NSE, glial fibrillar acid protein--GFAP and creatine kinase CK BB) in various structures of the postmortal brain of schizophrenic patients and those with senile dementia and Alzheimer's disease. The differences in the intensity and localization of these disorders in the above patients' groups have been detected. A previously unknown component of a pathological process in the brain, indicated by the decrease of the CK content and activity has been discovered. It is suggested that the decrease of the content of CK BB results in the development of energy deficit in the brain in patients with mental disorders.

We examined sera from six different groups of patients for CK-MB activity by means of two commercially available tests, an immunoinhibition method (E. Merck) and the CK-MB test as used with the aca (Du Pont). In the first group of patients (suspicion of myocardial infarction) the correlation between the two methods was good: r = 0.9191, y = 1.068x -- 0.888, x = 18.7 U/L, y = 19.0 U/L. In the second group, patients with high adenylate kinase activity, no interference was detectable on the aca, whereas the immunoinhibition method yielded falsely high CK-MB values. The third group consisted of persons with macro-CK-BB in their serum. In the immunoinhibition test these patients usually showed a high CK-MB:total CK ratio, whereas such results were rarely found for the aca. The fourth group, patients with a different electrophoretic mobility of their CK-isoenzymes (migration of an active band towards the cathode), were detected by the immunoinhibition method (high ratio of CK-MB to total CK), but not with the aca. In the presence of free CK-BB (group five) the immunoinhibition test resulted in "falsely" high CK-MB values, whereas CK-BB was retained on the column of the aca. In skeletal muscle diseases (group six) results by the two methods differed, values for CK-MB on the aca being much higher. It was demonstrated experimentally that this was due to CK-MM with altered surface charge.

Mucus samples obtained from 61 patients with different types of colon and rectal disease were assayed for CK isoenzyme fractionation. Twelve additional patients without evidence of such diseases were used as reference subjects. The diseases surveyed were hemorrhoids, proctocolitis, polyps, and cancer of the colon and rectum. The CK BB fraction was predominant in the mucus of the reference subjects and in mild cases of hemorrhoids and polyps. With inflammation of degeneration of the colonic mucosa, the present CK BB decreased and the percent of CK MM increased. It is speculated that the increased CK MM possibly derives from increased permeability of degeneration of the colonic wall, permitting plasma CK MM to admix with the mucus. Mucin CK isoenzyme fractionation may be useful in assessing the pathogenesis of colonic and rectal diseases, as a marker to monitor the efficacy of the therapeutic regimen, and as a technique for monitoring conversion of a premalignant process into a malignant one.

Uncontrollable hemorrhage accounts for a large proportion of total mortality in both civilian (31%) and military (47%) trauma victims. Hypothermia is a relatively safe method that could provide total body protection during hypovolemic shock and facilitate surgical intervention as a potentially life-saving procedure. This study tested the hypothesis that profound hypothermia and complete blood replacement in an established canine model, would facilitate resuscitative therapy from exsanguinating hypovolemic shock. Adult dogs were prepared for extracorporeal bypass using closed-chest peripheral cannulation under general anesthesia. Controlled hypotensive, hemorrhagic shock (mean arterial blood pressure < 50 mmHg) was induced for 30 min at normal temperature followed by temporary resuscitation using crystalloid infusion for approximately 10 min. Using our established procedure, the dogs were then cooled externally to 27 degrees C before initiating blood substitution with Hypothermosol (Cryomedical Sciences, Inc. Rockville, MD) via the extracorporeal pump. The heart was arrested during further cooling to below 10 degrees C and Hypothermosol was recirculated for 2 hr, with (3 dogs) or without (5 dogs) 1 hr of circulatory arrest. During rewarming the animals were autotransfused, weaned from the pump, and allowed to recover. All dogs (n = 8) survived, all but one with complete neurologic recovery: blood chemistry samples examined immediately after the procedure showed significant differences (p < 0.05) in only a few parameters, including creatine kinase (CK-BB and CK-MB), compared with the previous group of control dogs. The consistent survival of dogs showing apparently normal neurologic, physiologic, and biochemical recovery supports the concept that profound hypothermia using a protective hypothermic blood substitute could provide time for therapeutic resuscitation of currently intractable trauma cases.

Many biological substances are commonly used as markers for malignant neoplasms, but no single marker with high specificity and sensitivity has been found for cancer to date. In this study we evaluated simultaneously the serum levels of five biomarkers of malignancy: phosphohexose-isomerase (PHI), creatine kinase isoenzyme BB (CK-BB), alpha 1-acid glycoprotein (AAG), beta 2-microglobulin (BMG), and ferritin. In 89 female patients with breast lesions, we identified 30 benign lesions, 32 primary breast cancers, and 27 metastatic breast cancers (pulmonary and/or bone metastases). Each marker was assayed individually and in a combination and was compared with other markers. The results revealed that in benign lesions only 7% had PHI values higher than our cut-off limit value, while 3% had elevated values of AAG, BMG, and ferritin. In primary breast cancer we discovered pathological values of CK-BB and AAG in 71%, of PHI in 69%, of BMG in 50%, and of ferritin in 47%. Metastatic disease was associated with elevated values in 88% of CK-BB, in 70% of PHI and AAG, and in only 55% of BMG and ferritin. Combined pathological values for primary and metastatic breast cancer were 79% for CK-BB, 71% for AAG, 70% for PHI, and only 55% for BMG and ferritin. These data were assessed by the Student t test, which revealed for each marker a significant capacity (P less than 0.01) to discriminate between benign lesions and neoplastic diseases. The same capacity to distinguish between primary and metastatic cancer was obtained by the simultaneous use of three markers (CK-BB, PHI, and AAG).(ABSTRACT TRUNCATED AT 250 WORDS)

A biochemical marker of brain cell damage, the BB-isozyme of the intracellular enzyme Creatine Kinase (CK), was used to evaluate any possible injury to the brain, caused by an operation for a ruptured intracranial aneurysm (SAH). CSF-CK BB was assessed before and at intervals after operation in a series of 60 patients, aged 29-71 (mean 51 years) operated on for intracranial aneurysms, all but one after SAH. The m/f ratio was 18/42. 35 of the 60 patients were operated on acutely, i.e. within 72 hours after the SAH. CK BB was determined as CKB-activity after immunological inactivation of CKM. Normally there should be almost no detectable enzyme activity in the CSF. The pre-operative CK BB-activity was 0.01+ -0.01 mikrokatal in the patients in Hunt & Hess grade I who were operated on>> 7 days after their SAH, and 0.05+ -0.04 in those operated on acutely, probably still reflecting the effects of the SAH on the brain. The mean per-operative CK BB increase was 0.11+ -0.17 for patients who had an uneventful postoperative course, compared to 0.39+ -0.49 for those showing some degree of immediate postoperative deterioration. This difference is significant at the 1% level. 52 of the 60 patients showed a rise of CK BB after operation. The mean increase for those patients operated upon in a good state and without any complication or postoperative deterioration was 0.02+ -0.03 mikrokatal, which could therefore be considered as a "normal" or acceptable elevation.(ABSTRACT TRUNCATED AT 250 WORDS)

BACKGROUND

Early life-threatening cardiotoxicity and cardiac death have been reported after hematopoietic stem cell transplantation (HSCT). The purpose of the current study was to evaluate cardiac toxicity of conventional chemotherapy followed by HSCT with cardiac markers: heart-type fatty acid binding protein (H-FABP), glycogen phosphorylase BB (GPBB), high sensitive C reactive protein (hsCRP) cardiac troponin I, (cTnI), creatine kinase MB (CK-MB mass) and myoglobin.

METHODS

A total of 20 children who underwent HSCT for malignant and non-malignant diseases were included in this study. Blood samples were collected from all patients in 0th, 7th and 21st day for evaluating these cardiac biomarkers. The patients' echocardiography was assessment before and after one-month of HSCT.

RESULTS

Serum 21st H-FABP level was significantly higher when compared with the 0th day H-FABP level (P < 0.05) . 7th day hsCRP level was significantly higher than 0th and 21st day levels (P < 0.05). Interestingly, 7th day GPBB level was significantly lower than 0th and 21st day levels (P < 0.05). Myoglobin, CK-MB mass and cTnI biomarkers remained within the reference range in all patients.

CONCLUSIONS

This study showed that H-FABP and hsCRP both seem to be promising markers for evaluation of cardiotoxicity in HSCT process and probably superior to GPBB, cTnI, CK-MB mass and myoglobin.

Biochemical analyses of the brain of Cichlid fish larvae, exposed during their very early development for 7 days to an increased acceleration of 3g (hyper-gravity), revealed a decrease in brain nucleoside diphosphate kinase (NDPK) as well as creatine kinase (BB-CK) activity. Using high performance liquid chromatography (HPLC) the concentrations of adenine nucleotides (AMP, ADP, ATP), phospliocreatine (CP), as well as of nicotineamide adenine dinucleotides (NAD, NADP) were analyzed in the brain of hyper-g exposed larvae vs. 1g controls. A slight reduction in the total adenine nucleotides (TAN) as well as the adenylate energy charge (AEC) was found. In parallel a significant increase in the NAD concentration and a corresponding decrease in NADP concentration occurred in larva's hyper-g brains vs. 1 g controls. These results give further evidence for an Influence of gravity on cellular level and furthermore contribute to a clarification of the cellular signal-response chain for gravity perception.

Changes in creatine kinase (CK) and lactate dehydrogenase (LDH) isoform expression occur in residual tissue after myocardial infarction. It is unknown how these changes correlate with cardiac remodeling, contractile performance and efficiency. Rats were subjected to left coronary artery ligation (MI) or sham operation (sham). Left ventricular end-diastolic pressure (EDP) was measured in vivo 8 weeks later. Hearts were isolated, buffer-perfused (Langendorff) at constant pressure and isovolumetric left ventricular (LV) pressure-volume (PV) curves were recorded. LV PV areas (PVA) were calculated and related to oxygen consumption. Biopsies of intact left ventricular tissue were taken for biochemical measurements. Correlations between in vivo EDP and biochemical parameters were found: Total CK activity (r = -.47, p = .022), CK isoenzyme percentage for BB (r = +.57, p = .004), MB (r = +.54, p = .006) and CK-mito (r = -.51, p = .012), total creatine content (r = -.61, p = .002) and the ratio of LDH5/LDH1 (r = .49, p = .016). Correlations were also detected for left ventricular volume and PVAs at in vivo EDP demonstrating that the extent of CK and LDH system alterations correlate with the extent of LV dilatation and mechanical energy requirements. The slope of the MVO(2)-PVA relation decreased significantly with increasing values of in vivo EDP (r = -.68, p = 0.0003) indicating increased contractile ef.ciency. Improved efficiency correlated with the increase in fetal CK isoenzyme expression. Thus, contractile efficiency increases parallel to the extent of left ventricular dilatation and dysfunction. CK and LDH system changes in residual intact myocardium also occur proportional to LV dysfunction.

A human tumor cell line designated RMS-GR was established from an embryonal rhabdomyosarcoma. The monolayer cells were polygonal, round or spindle-shaped. The RMS-GR cell line became stable with a doubling time of 42 h. Tumorigenicity of the cells was confirmed by heterotransplantion into nude mice. Electron microscopic images showed typical cytoplasmic inclusion of aggregated intermediate filaments and myofibril-like thin filaments. The expression of desmin, vimentin, actin and human myoglobin was recognized by cytofluorometric analyses, and a large fraction of CK-MM and small fractions of CK-BB and MCK-1 isoenzymes were found. Chromosomal analysis showed that the modal chromosome number was consistently near triploid with structural abnormalities mostly involving chromosomes 1, 3 and 8, and additional unidentified markers. No alteration of chromosome 2 was observed. The RMS-GR cell line may provide a system to identify genes which are involved in the pathogenic mechanism of rhabdomyosarcomas, and to investigate the modulation of myogenic differentiation.

Certain proteins, such as inflammatory cytokines, that are released from injured or diseased organs are transported from the circulating blood through the blood-brain barrier (BBB) into the brain, and contribute to the pathogenesis of related central nervous system dysfunctions. However, little is known about the protein transport mechanisms involved in the central nervous system dysfunctions. The aims of the present study were to identify BBB-permeable protein(s) derived from liver and to clarify their transport characteristics at the BBB. After administration of biotin-labeled liver cytosolic protein fraction to mice in vivo, we identified 9 biotin-labeled proteins in the brain. Among them, we focused here on creatine kinase (CK). In vitro uptake studies with human brain microvessel endothelial cells (hCMEC/D3 cells) showed preferential uptake of muscle-type CK (CK-MM) compared with brain-type CK (CK-BB) at the BBB. Integration plot analysis revealed that CK-MM readily penetrated into brain parenchyma from the circulating blood across the BBB. The uptake of CK-MM by hCMEC/D3 cells was decreased at 4ºC and in the presence of clathrin- and caveolin-dependent endocytosis inhibitors. These results indicate that entry of CK into the brain is mediated by a transport system(s) at the BBB.

Creatine kinase brain isoenzyme (CK-BB) was determined in cerebrospinal fluid of 150 neonates by a newly developed immunoenzymatic assay. Newborns with a documented neurologic disorder (intraventricular hemorrhage, postasphyxial encephalopathy, central nervous system infection, or persistent periventricular intraparenchymal echodensities) showed markedly higher concentrations of immunoreactive CK-BB than did the normal newborns or those with subarachnoid hemorrhage. In neonates with seizures the data suggest that the underlying neurologic disorder accounts for the higher CK-BB values and not the seizures per se. High concentrations of CK-BB in the neonatal period were followed by poor short-term outcome.

Radioimmunoassay-determined myelin basic protein (MBP) shed to CSF during active demyelination, has been found to be a useful but non-specific test for MS. CSF creatin kinase BB (CK-BB), as measured by radioimmunoassay, is increased in a variety of neurological diseases, and has been considered a useful indication of brain damage but not of demyelinating diseases. Taking into account that the mean concentration of CSF CK-BB should not be increased in patients during the acute phase of MS, we suggest that the CSF MBP/CK-BB ratio could be a more specific index to demyelination than CSF-MBP alone. We also defined a laboratory demyelination pattern (CSF MBP greater than mean control MBP + 2 S.D. and CK-BB less than MBP). CSF levels of MBP and CSF levels of CK-BB were determined by radioimmunoassay in 232 patients with several neurological disorders and 33 control subjects. Patients diagnosed as having MS with clinical exacerbation had significantly higher values of CSF-MBP/CSF-BB ratio than control subjects. Our study showed a significant presence of demyelination pattern in CSF of patients with MS. We conclude that the CSF MBP/CK-BB ratio and the CSF demyelination pattern may be new and reliable tests for the diagnosis of MS.

Hybrid cells derived from rat L6 myoblasts and mouse primary fibroblasts (M x F hybrids), as well as those derived from rat L6 myoblasts and mouse primary myoblasts (M x M hybrids), were examined for their ability to engage in myogenesis as judged by muscle fiber formation plus the expression of skeletal muscle myosin and creatine kinase (CK). Of 172 primary hybrid colonies scored, 59% were myogenic in the M x F fusion and 97% exhibited muscle fiber formation in the M x M fusion. Individual hybrid clones from each cross were isolated, expanded and analyzed for myogenic capabilities as well. All three M x M and all ten M x F isolated clones exhibited preferential elimination of mouse chromosomes. Nonetheless, all were capable of fusing spontaneously and of elaborating skeletal muscle myosin and CK. The three M x M hybrids expressed only MM-CK whereas nine out of ten M x F hybrids produced all three CK isoenzymes (MM, MB, BB). These results suggest that M X M hybrids express CK patterns reminiscent of the rat L6 parental cells while M X F hybrids apparently mimic mouse muscle fiber CK patterns. Various models are discussed which address these phenomena.

Prolonged strenuous exercise is associated with the appearance of biomarkers of cardiac cell damage and a decline in cardiac function during recovery. Few studies have assessed repeated bouts of prolonged exercise and whether this results in further biomarker accumulation and greater dysfunction. Further, it may be useful to describe the changes in a range of biomarkers that may provide additional insight into the clinical significance of cardiac biomarker release. Four highly trained cyclists completed the 4800 km Race Across America (RAAM) in 7 days. Venous blood samples and echocardiograms were taken prior to, every 24 hours during and immediately after the RAAM. Venous blood was analysed for cardiac troponin I (cTnI), creatine kinase MB (CK-MB), fatty acid binding protein (HFABP), glycogen phosphorylase BB (GPBB) and N-Terminal Brain Natriuretic Peptide (NTproBNP). Echocardiograms allowed analysis of septal, left ventricular free wall and right ventricular free wall tissue velocities during systole and diastole. Before the RAAM cTnI levels were below the assay detection level (0.02 ng.ml⁻¹). In three riders cTnI peaked on day one (0.03 ng.ml⁻¹) and returned below detection levels post race. In the 4th rider cTnI peaked on day 5 (0.08 ng.ml⁻¹) and was still elevated post-race. Both CK-MB and H-FABP were increased during the RAAM in all 4 cyclists. In three riders H-FABP peaked on day one (3.49 to 5.09 ng.ml⁻¹) and declined over the rest of the RAAM. In the final rider H-FABP peaked on day two (5.90 ng.ml⁻¹) and then dropped back to baseline by the post-RAAM assessment. Interestingly, changes in H-FABP mirrored, temporally, changes in CK-MB in places and this may reflect an association with skeletal muscle damage. Data for GPBB value to (2.9 - 149.6 ng.ml⁻¹) and NTproBNP value to (27.3 - 310.0 ng.L⁻¹) were variable but again was elevated in all riders during the course of the RAAM. Changes in ventricular wall tissue velocities were minor and not cumulative. Peak atrial diastolic tissue velocity in the left ventricular free wall increased (P < 0.05) from 11 to 18 cm.s⁻¹ over the last two race days but this did not significantly impact the ratio of early to late diastolic wall motion. Cardiac biomarkers were elevated during the completion of the RAAM in all 4 cyclist but changes were not cumulative which suggest that the hearts of the cyclists coped well with the extreme cardiac work demanded by this ultra-endurance exercise challenge.

The Creatine kinase (CK) SYSTEM represents key in a power exchange mediators the structure capable to plural interactions with the majority energy making (Glycolysis and mitochondriuns) and energy consuming (ATPases) structures at use of one multifunctional metabolits--creatine and providing transport macroergs inside a cell. Mitochondrions CK provides synthesis creatine phosphates (CP) from cytoplasmic creatine and energy mitochondriums ATP. CP energetically also is structurally more favourable than ATphi. The MM, MB and BB isoforms provide splitting Kphi and synthesis ATphi for M-ATPases, Ca-ATPases and Na-K-ATPases accordingly. Questions of regulation of activity of enzyme, both in ontogenesis, and in blood are discussed.

In a family of Italian origin, we found four members with a considerable activity of creatine kinase inside their erythrocytes. All other clinical and hematological findings were normal. The enzyme anomaly seems to be inherited in the autosomal mode. The creatine kinase CK) activity in freshly drawn blood was about 12 U/g Hb. The activity was higher in young red cells than in older ones. Studies with specific antibodies against human CK isoenzymes revealed the CK activity in the probands' red cells to be due to about 90% to the BB-isoenzyme normally found in brain and nerve tissue. The presence of CK in the erythrocytes does not seem to have any consequences for the energy metabolism of the cells. Creatine concentration was slightly elevated, but creatine phosphate could not be detected.

Serum creatine kinase (CK) and creatine kinase-MB isoenzyme (CK-MB) activities were studied prospectively in serial blood samples obtained from 23 perinatally asphyxiated negroid newborns and 12 healthy controls during the first 100 h of life. The asphyxiated infants had significantly elevated mean CK and absolute CK-MB but no fractional CK-MB activities. Peak mean CK and CK-MB values (U.l-1) were 789.17 (+/- 220), P less than 0.01 and 16.36 (+/- 3.0) P less than 0.001 respectively at the 6-8 post-partum period. The healthy controls, on the other hand, showed a steady decline in the activities of these enzymes from birth. The vaginally and operatively delivered asphyxiated infants showed significantly higher CK and CK-MB activities than their respective non-asphyxiated controls, but no increase in fractional CK-MB was recorded in any of the groups. The elevation in absolute and fractional CK-MB 42.0 U.l-1 (5.1%) in respect of the infants with TTI (transient tricuspid incompetence) was significant (P less than 0.05) when compared with the controls with features of TTN (transient tachypnoea of the newborn) in the 6-8 h post-partum period. One of the infants with TTI at autopsy had hypoxic myocardial injury. The specificity of CK-MB, as a marker of myocardial injury in asphyxiated negroid neonates, is plausible but remains uncertain. Until the lack of rise of CK and CK-MB in healthy negroid newborns is confirmed in a larger series with further studies on MM, BB and MB isoenzymes, caution is urged in the interpretation of elevated CK and CK-MB activities in asphyxiated negroid newborns.

Increased activity of the MM isoenzyme of creatine kinase (CK) was found in plasma from Bar Harbor dystrophic mice of the 129/ReJ dy/dy strain when compared to the findings in non-dystrophic controls. Total plasma CK activity was only slightly increased in dystrophic animals but plasma from both normal and dystrophic mice showed large amounts of the BB isoenzyme of creatine kinase. This isoenzyme was also found to be present in mouse platelets. It is concluded that BB isoenzyme from platelets could have obscured the contribution of muscle CK to the total plasma CK activity and the CK isoenzyme quantitation is needed to evaluate muscle disorders in rodents.

1. A monoclonal antibody (subclass immunoglobulin G1) has been raised against human brain-type creatine kinase (CK-BB). This antibody did not cross-react with either muscle-type creatine kinase (CK-MM) or heart-type creatine kinase (CK-MB). 2. The binding constant measured with native antibody was 6 X 10(8) M-1. In the presence of 2mM-dithiothreitol this constant was some 40-50-fold greater. 3. Partial reduction and alkylation showed that the increased binding was due to a direct effect on the antibody and was associated with concomitant cleavage of the heavy-heavy interchain disulphide bonds. The binding constant measured with Fab' fragments produced from reduced and alkylated antibody was similar to that shown by the native, unreduced antibody. 4. The molecular weight of the complex found in the absence of mercaptans was consistent with one antibody and one CK-BB molecule, whereas the molecular weight estimated with reduced and alkylated antibody was consistent with a complex of two antibodies and two CK-BB molecules. 5. It is proposed that mercaptans increase the flexibility of the hinge region of the antibody molecule, allowing the formation of a higher-order complex with increased avidity for the CK-BB dimer.

A bioluminescent assay based on the firefly luciferase reaction has been used for determination of creatine kinase activity in CSF. Activities as low as 0.1 U/L can be measured. The coefficient of variation at an activity level of 0.3-0.4 U/L was between 5 and 6%. The assay conditions optimized for serum specimens can be used for CSF. The adenylate kinase activity is almost completely inhibited, which simplifies the procedure. The creatine kinase/(CK) isoenzyme distribution was obtained using the bioluminescent assay in combination with immunoinhibition or ion exchange chromatography. All specimens contained both MM and BB activity, but no MB was found. The study indicates that the bioluminescent assay is useful in the determination of CK isoenzymes in CSF. The clinical importance of the observed CK levels will be reported in a separate communication.

Histological studies were correlated with developmentally regulated isoenzyme transitions of creatine kinase (CK) in free soleus muscle grafts in mice, 1-25 days after operation. One series of muscles was treated with Marcaine, a myotoxin, in order to promote subsequent regeneration of the muscle. Another set of muscles was mock-treated. In both series of experiments, CK-BB and hybrid CK-MB isoenzymes reappeared transiently as the myoblasts fused and the muscle regenerated. CK isoenzyme transitions were somewhat more pronounced in Marcaine-treated muscle. Nine days after grafting the muscle, specific CK-MM was again present almost exclusively. In this regenerating system, therefore, isoenzyme transitions correlate with histologically detectable regenerative changes, recapitulating the events of embryonic muscle development.

BB isoenzyme of creatine phosphokinase (CK-BB) obtained from human brain-extract changes its electrophoretic mobility after incubation in human serum at 37 degrees C. No change of electrophoretic mobility of CK-BB is observed after incubation in isotonic saline. We have shown by means of immunoprecipitation with specific antibodies that the structure of CK-BB is not changed. These findings are supported by other authors and make the diagnostic value of electrophoretic separation of CK isoenzymes doubtful as after a 3-h incubation CK-BB migrates similarly to CK-BB and consequently may be misinterpreted.

Creatine kinases (CK) play a prominent role in cell energy distribution through an energy shuttle between mitochondria and other organelles. Human brain CK was cloned and overexpressed in COS-7 cells. We then deleted His-65 and/or Pro-66 situated near the center of a flexible loop as shown by X-ray crystallography on mitochondrial and cytosolic CK. The DeltaH65 mutant had nearly the same affinity for its substrates as wild isoenzyme, but its stability was very low. Unlike DeltaH65, DeltaH65P66 had a eightfold decreased affinity for creatine phosphate and was unable to dephosphorylate cyclocreatine phosphate. Our results demonstrate that, despite an overall similar shape of the proteins, this loop accounts for some subtle differences in isoenzyme functions.