Rhizopus Acid Proteinases (Rhizopus-pepsins): Properties and Homology with Other Acid Proteinases

Abstract

Commercial acid proteinase from Rhizopus chinensis has been further purified by isoelectric focussing. Two forms of the enzyme (I and II) with very similar properties have been obtained. Like other fungal acid proteinases they activate trypsinogen at pH 3.4 and are inhibited by diazoacetyl norleucine methyl ester. Because of the lability of the label it has not been possible to isolate the active site peptide from a peptic digest. N-terminal amino acid sequences were determined for Rhizopus enzyme I (NH2∙Ala∙Gly∙Val∙Gly∙Thr∙Val∙Pro∙Asx∙Thr), for Rhizopus enzyme II (NH2∙Ala∙Gly∙Val∙Gly∙Thr∙Val∙Pro), and for penicillopepsin (NH2∙Ala∙Ala∙Ser∙Gly∙Val∙Ala∙Thr∙Asn∙Thr∙Pro∙Thr). The similarity in enzymic properties and in the sequence suggests that the Rhizopus enzymes are homologous with penicillopepsin and hence also with mammalian pepsins and calf chymosin.