Further studies on the primer formation for glycogen biosynthesis in rat heart

Using selected incubation conditions we have identified intermediate steps, between the first glucose transferred to protein and the appropriate substrate for glycogen synthase. Mn2+ stimulates the addition of the first, and probably, the second glucose molecule to the acceptor protein but inhibits further elongation. In the presence of Mn2+ only one radioglucosylated protein band of M(r) 42 kDa was evident. In the absence of Mn2+, two bands of 60.7 and 64.6 kDa were obtained indicating elongation of the glucan chains. After Glc6P addition a family of glucosylated proteins with higher M(r) was obtained, as reported previously. Mn2+ inhibition of the second step, is reversed by PMSF+Glc6P addition. Under these conditions a family of radioglucosylated protein bands with M(r) far in excess of 42 kDa, similar to that obtained without Mn2+, was obtained. Therefore, two different transglucosylating activities were necessary, at least, to prepare the appropriate substrate for glycogen synthase. Based on these observations the model we proposed earlier for glycogen biogenesis is modified. The original "Glycogen Initiator" implies at present two enzymatic activities, Glycogen Initiator 1 (activated by Mn2+) and Glycogen Initiator 2 (inhibited by Mn2+).