EXTRACELLULAR ASPARTIC PROTEINASES FROM CANDIDA-ALBICANS, CANDIDA-TROPICALIS, AND CANDIDA-PARAPSILOSIS YEASTS DIFFER SUBSTANTIALLY IN THEIR SPECIFICITIES

Extracellular aspartic proteinases have been implicated for some time as virulence factors associated with Candida opportunistic fungal infections. Our present knowledge of the enzymatic properties of these proteinases is rather limited. Information on their substrate specificity is important for understanding their roles in invasive Candida infections. We have isolated aspartic proteinases from each of the three Candida yeasts, Candida albicans, Candida tropicalis, and Candida parapsilosis, and investigated the specificities of these proteinases using a library of synthetic substrates and testing inhibition by pepstatin A. The specificities of these aspartic proteinases are different from those of major human proteinases, including gastric pepsins, renal renin, and cathepsin D. For the peptide substrate, Lys-Pro-Ala-Leu-Phe*Phe(p-NO2)-Arg-Leu, the values of k(cat)/K-m were 2.95 x 10(6) M(-1) s(-1) for cleavage by Candida albicans proteinase, 1.60 X 10(6) M(-1) s(-1) for cleavage by Candida tropicalis proteinase, and 0.59 x 10(6) M(-1) s(-1) for Candida parapsilosis proteinase. Substantial differences in specificity among the Candida yeast proteinases were identified. For example, Candida tropicalis shows large changes in the k(cat)/K-m value depending on the acidobasic character of the residue occupying the P-2 position (1.6 X 10(6) M(-1) s(-1) for Leu, 0.47 X 10(6) M(-1) s(-1) for Lys, and 0.05 x 10(6) M(-1) s(-1) for Asp at P-2, respectively). Candida parapsilosis by comparison is tolerant of these substitutions at P-2 and is highly restrictive at position P-4. The comparison of sequences of these proteinases, taken together with the kinetic data, suggests the participation of as yet unidentified residues of aspartic proteinases in forming the specificity binding pockets.