1. 2-Methyl-1, 4-naphthoquinone (K3) reacted with sulfhydryl group of protein such as bovine serum albumin and papain to form a thioether linkage at 3-position of K3 as well as sulfhydryl groups of low molecular compound. 2. K3 bound to detectable sulfhydryl groups in the active site of native papain with high specificity and its binding was not cleaved by the addition of cysteine.
