Although the mesquite plant (Prosopis velutina) is not as widely distributed as some other allergenic species, its pollen can induce serious pollinosis in areas where it is localized. We previously isolated and characterized a peptidase from mesquite pollen with trypsin-like specificity (peptidase Imes) (Matheson, N., Schmidt, J., and Travis, J. (1995) Am. J. Respir. Cell Mol. Biol. 12, 441-448). Now we have characterized a second enzyme with specificity for hydrophobic residues (mesquite pollen peptidase IImes). This enzyme has a molecular mass near 92 kDa and activity that was not affected by reducing or chelating agents but was inhibited by specific synthetic serine proteinase inhibitors and the aminopeptidase inhibitor bestatin. However, it was not inhibited by human plasma proteinase inhibitors, nor did it inactivate any of those tested. The enzyme possessed amidolytic activity against p-nitroanilide substrates most effectively after alanine residues and also displayed aminopeptidase activity against non-p-nitroanilide peptides with a preference for phenylalanine. This specificity for hydrophobic amino acid residues was corroborated by inhibition studies with chloromethyl ketone and organophosphonate inhibitors. More interesting from a physiological point of view is that the bioactive peptides, angiotensins I and II and vasoactive intestinal peptide, were also hydrolyzed rapidly, indicating an ability of peptidase IImes to act also as an oligopeptidase. Because these bioactive peptides play a role in the inflammatory responses in allergic asthma, our data suggest that the purified mesquite pollen peptidase IImes may be involved in the degradation of neuro- and vasoactive peptides during pollen-initiated allergic reactions.