Incubation of rabbit skeletal myosin with 1 to 3 mM D‐glucose 6‐phosphate over a period of several hours resulted in the inhibition of the K+‐ and actin activated‐ATPase activities. Substrate ATP (0.5 ‐ 3 mM final concentration) protected the myosin against the loss of ATPase activity as induced by glucose 6‐phosphate. This was also found for ADP. When the myosin was incubated with 3 mM [3H] labeled glucose 6‐phosphate for 28 h. up to one mole of glucose 6‐phosphate was incorporated per 4.7 × 105 g of myosin. A significant reduction in the labeling occurred in the presence of ATP. The labeling was limited to the heavy chain region as judged by gel electrophoresis which resolved the heavy and light chain components of myosin. The non‐enzymatic glycation of myosin by glucose 6‐phosphate is probably the primary cause for the observed loss of the ATPase activity of myosin. This effect may also occur physiologically modifying the activity of muscle contractile proteins particularly during prolonged hyperglycemia.

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A bovine cardiac actin-tropomyosin-troponin complex was phosphorylated in the presence of [gamma-32P]ATP, Mg2+, adenosine 3',5'-monophosphate (cyclic AMP), and bovine cardiac cyclic-AMP-dependent protein kinase. Approximately 81% of the [32P]phosphate incorporated was identified as phosphoserine and phosphothreonine. Gel electrophoresis studies showed that 55% of the [32P]phosphate was associated with the inhibitory component of troponin (Tn-I) and 24% with a protein resembling the tropomyosin-binding component of troponin in the actin complex, respectively. The phosphorylation of Tn-I in the actin complex was inhibited 30% when Ca2+ was increased from 0.1 to 50 muM, but phosphorylation of other components was not affected by increasing Ca2+ concentration. Half-maximal calcium activation of the ATPase activity of reconstituted actomyosins made with the [32P]phosphorylated cardiac actin complex and cardiac myosin was shifted to Ca2+ values higher than those of actomyosins made with the nonphosphorylated actin complex.

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