S-PI(Pepstatin Ac)-insensitive acid proteinase was found in a culture filtrate of Scytalidium Hgnicolum ATCC 24568 and designated as acid proteinase C. Acid proteinase C was purified and about 320 mg of the purified enzyme was obtained from 41.5 liters of culture filtrate, a recovery of 0.9%. Acid proteinase C was monodispersive by physicochemical criteria such as ultracentrifugal analysis and disc electrophoresis.
Acid proteinase C was most active at pH 2.0 toward casein, and stable in the pH range of 3 to 5 under treatment at 37°C for 20 hr. The molecular weight was 4.06×10⁵ by sedimentation equilibrium analysis, and 3.6 × 10⁵ by Andrew's gel nitration method. A subunit structure was suggested by SDS-PAGE.
Acid proteinase C was characterized by the following points: (1) the proteolytic activity was not inactivated by any of such acid proteinase inhibitors as S-PI, DAN and EPNP; (2) the molecular weight was extraordinarily high in comparison with those of other proteinases; (3) acid proteinase C was inactivated weakly by alpha-MAPI, a potent inhibitor of serine- and thiolproteinases; (4) acid proteinase C hydrolysed at a high rate MAPI-analogous peptide substrates (Suc-Phe-Arg-Ala-Phe-NPhNO2: k_cat/Km, 5.8 × 10⁴M^-1 • sec^-1).

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