Compartmentalized ATP synthesis in skeletal muscle triads (original) (raw)

Isolated skeletal muscle triads contain a compartmentalized glycolytic reaction sequence catalyzed by aldolase, triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, and phosphoglycerate kinase. These enzymes express activity in the structure-associated state leading to synthesis of ATP in the triadic junction upon supply of glyceraldehyde 3-phosphate or fructose 1,6-bisphosphate. ATP formation occurs transiently and appears to be kinetically compartmentalized, i.e., the synthesized ATP is not in equilibrium with the bulk ATP. The apparent rate constants of the aldolase and the glyceraldehyde-3phosphate dehydrogenase/phosphoglycerate kinase reaction are significantly increased when fructose 1,6bisphosphate instead of glyceraldehyde 3-phosphate is employed as substrate. These observations suggest that fructose 1,6-bisphosphate is especially effectively channelled into the junctional gap. The amplitude of the ATP transient is decreasing with increasing free [Ca*+] in the range of 1 nM to 30 FM. In the presence of fluoride, the ATP transient is significantly enhanced and its declining phase is substantially retarded. This observation suggests utilization of endogenously synthesized ATP in part by structure associated protein kinases and phosphatases which is confirmed by the detection of phosphorylated triadic proteins after gel electrophoresis and autoradiography. Endogenous protein kinases phosphorylate proteins of apparent M, 450 000, 180 000, 160 000, 145 000, 135 000, 90 000, 54 000, 5 1 000, and 20 000, respectively. Some of these phosphorylated polypeptides are in the M, range of known phosphoproteins involved in excitation-contraction coupling of skeletal muscle, which might give a first hint at the functional importance of the sequential glycolytic reactions compartmentalized in triads.