Interactions between Native and Chemically Modified Subunits of Matrix-Bound Glycogen Phosphorylase

Phospho-dephosphohybrids of rabbit skeletal muscle phosphorylase (EC 2.4.1.1; alpha-1,4-glucan: orthophosphate glucosyl transferase) have been prepared and stabilized by attachment to Sepharose activated by cyanogen bromide. They can be distinguished from phosphorylase a by their sensitivity to inhibition by glucose-6-phosphate and activation by adenosine 5'-monophosphate. Stable hybrids have also been formed between phosphorylase subunits containing the active cofactor pyridoxal-phosphate and inactive analogs (pyridoxalphosphate monomethylester or the corresponding reduced compounds). After complete dissociation to monomers, the Sepharose-bound phosphorylase had a residual activity of less than 3% of that of the original matrix-bound dimeric enzyme. The hybrid enzyme is composed of a potentially active subunit containing pyridoxal-phosphate and an intrinsically inactive subunit carrying the analog, and it had half the activity of the original dimeric enzyme. Thus, the interaction of the inactive subunit with matrix-bound phosphorylase monomers elicited activity in the monomers.