Abstract
The number and reactivity of the sulfhydryl (SH) groups of potato phosphorylase [EC 2. 4. 1. 1] were investigated, in comparison to those groups in muscle phosphorylase b. Potato phosphorylase was found to contain 17.2 moles of total half-cystine residues per 200000g of protein; all of which seemed to exist in the form of the SH group. Reactivity of SH groups in the native state with various SH reagents was low. Inactivation with p-chloromercuribenzoate and iodoacetamide proceeded gradually. No inactivation was observed with 5, 5'-dithiobis-(2-nitrobenzoic acid), N-ethylmaleimide and iodoacetic acid. The enzyme apparently did not dissociate into subunits upon reacting with SH reagents. The difference in quarternary structure may be directly related to differing control mechanisms in potato and rabbit muscle phosphorylases, although their other properties are surprisingly similar.
