The molecular basis of the heterogeneity of the MM isozyme of rabbit muscle creatine phosphokinase

Abstract

Purified rabbit muscle creatine phosphokinase MM isozyme was resolved into a pattern of six subbands by starch gel electrophoresis at pH 6.3 in a Tris-citrate buffer system. Following urea dissociation and denaturation, urea-starch gel electrophoresis resolved six distinct, enzymatically inactive subunits of molecular weight 43 000 daltons as determined by sodium dodecyl sulfate-acrylamide gel electrophoresis. This indicates that six different subunit types are involved in rabbit MM isozyme subbanding. The six subbanded pattern obtained following renaturation and reassociation from urea was identical to that exhibited by the purified, untreated MM isozyme. The exact nature of the subbanding is not clear; but it does not appear to be due to sulfhydryl group interactions or to alternative conformations of a single M-type subunit. Rather, it appears to be due to conformational differences resulting from alterations in primary structure, deamidation or ligand heterogeneity.