Studies on proteinases from the digestive organs of sardine. II. Purification and characterization of two acid proteinases from the stomach.

Two fish acid proteinases designated acid proteinase I and II were found and isolated by (NH4)2SO4 fractionation, CM-cellulose chromatography and gel filtration on Sephadex G-100. The final preparations were judged nearly homogeneous by multiple criteria. The molecular criteria. The molecular weights of the enzymes I and II were determined by the sedimentation equilibrium method to be 37 000 and 33 400, respectively. The sedimentation coefficients (S0 20, w) were 3.06 and 3.09, respectively. Enzymes I and II contained similar amino acid composition except for the contents of histidine, arginine, threonine, serine and proline. Enzymes I and II differed from each other in optimal pH and stability at pH 7. Each enzyme could scarcely hydrolyze a synthetic pepsin substrate, N-acetyl-L-phenylalanyl-3,5-diiodo-L-tyrosine (APDT). Both of the enzymes were inhibited by acid proteinase specific reagents: pepstatin, diazoacetyl-DL-norleucine methyl ester (DAN), 1,2-epoxy-3-(p-nitrophenoxy) propane (EPNP) and p-bromophenacyl bromide. These results indicate fish enzymes are similar to mammalian pepsin and microbial acid proteinases in their active site structure having two different carboxyl groups, although they differ in regard to a number of molecular and enzymatic properties.