Cleavage specificity of cucumisin, a plant serine protease.

Cucumisin was isolated from prince melon sarcocarp by means of a simple purification procedure. Serine protease inhibitors such as soybean trypsin inhibitor, ovomucoid, and aprotinin had no effect on the enzyme activity. alpha 2-Macroglobulin showed 38% inhibition of the original caseinolytic activity of cucumisin. The favorable synthetic substrates for cucumisin were Glt-Ala-Ala-Pro-Leu-pNA and Suc-Ala-Ala-Pro-Phe-pNA. The constant (kcat/Km) for Suc-Ala-Pro-Ala-pNA was found to be 30 times greater than that for Suc-Ala-Ala-Ala-pNA. The substrate specificity of cucumisin for oligopeptides and proteins was shown to be broad.