Characterization of different forms of kinase FA from rabbit skeletal muscle

Amongst the several cyclic AMP-and Ca2+-independent synthase kinases that are reported by several laboratories, the kinase FA is unique in having a bifunctional nature: it can also promote the conversion of the inactive ATP,Mg-dependent protein phosphatase to its active form. By doing so, it produces an active multisubstrate phosphatase which reverses the major protein phosphorylations that control glycogen metabolism. In rabbit skeletal muscle, two forms of kinase FA can be distinguished, which seem to interconvert into one common bifunctional catalytic unit. It is proposed that the two molecular forms either reflect the existence of a regulatory subunit which dissociates during the purification, or suggest a reversible modification of the bifunctional catalytic unit. In order to serve a useful physiological role in the regulation of glycogen metabolism, the enzyme has to select between its two opposite activities in any given physiological condition, and the putative regulatory subunit or modification of the catalytic unit could play a fundamental role in this modulation. In vitro experiments have provided us with two proteins that are suitable candidates to discriminate between the two activities in FA: the heat stable phosphatase inhibitor-1 and the phosphatase modulator protein. Both can prevent the expression of the protein phosphatase activity; however, a well defined amount of modulator protein is absolutely required for an efficient activation of the FC-enzyme by FA, but an excess [M] decreases the rate as well as the extent of activation. This means that excess modulator protein can block the phosphatase activating capacity of FA. The same, or even higher, concentrations of modulator (or inhibitor-1) have absolutely no effect on the synthase kinase activity of the FA protein. No effector of this synthase kinase activity has yet been found, although by its shunting away from the phosphatase activation, more of the FA could be effective as a synthase kinase.

Regulation of cardiac glycogen synthase

In perfused rat hearts insulin can activate, and catecholamines can inactivate, glycogen synthase (EC 2.4.1.11); the magnitude of each hormonal response is magnified if tissue glycogen levels are depleted. Both beta-adrenergic and alpha-adrenergic agonists inactivate insulin-stimulated and basal glycogen synthase, with each promoting the same extent of inactivation in both circumstances. In this system beta-adrenergic agonists act via cyclic AMP (cAMP), and alpha-adrenergic agonists via Ca2+, whereas insulin action appears to be independent of either cAMP or Ca2+. The action on cardiac glycogen synthase by the physiological catecholamine epinephrine is apparently mediated by the concomitant interaction with both alpha and beta receptors; interaction with each is mediated by their separate second messenger systems, which combine to produce the end physiological response.

Cyclic adenosine monophosphate dependent and independent phosphorylation of sarcolemma membrane proteins in perfused rat heart

This study was initiated in order to elaborate further on the mechanism by which epinephrine modulates cardiac function via protein phosphorylation. A membrane fraction has been isolated from freeze-clamped perfused rat heart that contains two phosphoproteins. These proteins have molecular weights of 36,000 (A protein) and 27,000 (B protein). The phosphorylation of the A protein occurs during the equilibration of the heart with inorganic [32P]phosphate. The phosphorylation of the B protein occurs in response to epinephrine. The A and B proteins are apparently identical with two phosphoproteins in enriched preparations of sarcolemma. The protein of the sarcolemma preparation equivalent to the A protein is phosphorylated in vitro by both cAMP-independent and cAMP-dependent protein kinases. The phosphorylation of the protein of the sarcolemma preparation equivalent to the B protein is catalyzed by the cAMP-dependent protein kinase. Thus the patterns of phosphorylation of these proteins in vivo and in vitro are compatible. The phosphorylation of the B protein has been documented in vitro to modulate calcium transport (Will, H., et al. (1973) Acta Biol. Med. Ger. 31, 45-52), but the response to epinephrine in the perfused heart is not apparently coordinated with the catecholamine-induced inotropic effect.