Possible mechanisms of inhibition and activation of rat N-acetyltransferase (EC 2.3.1.5.) by cations

Abstract

The possible mechanisms of inhibition and activation of various cations on rat pineal N-acetyltransferase (NAT) were elucidated. Copper was found to be a partial mixed noncompetitive inhibitor of NAT with respect to both substrates and this inhibition can be considered to result primarily from impairment of tryptamine (serotonin) binding to the enzyme. Both calcium and magnesium were found to activate NAT by a similar mechanism, with calcium being more effective than magnesium. It appears that the activation results from cation binding to the enzyme causing an increased affinity of tryptamine (serotonin) for binding to NAT and therefore enhancing catalytic activity. The monovalent cations, potassium and sodium, activated NAT by a similar mechanism which differed from the caused by the divalent cations. It can be suggested that tryptamine (serotonin) binds to the enzyme followed by the monovalent cation which enhances binding of the acetyl donor and thereby promotes catalysis. At high potassium or sodium concentration the affinity of acetyl coenzyme A for NAT begins to decrease suggesting that excess monovalent cations can be inhibitory and may represent an endogenous regulatory mechanism controlling in vivo NAT activity. It is possible that certain cations may be involved in regulation of melatonin synthesis although the physiological significance of such modulation remains unknown.

Dietary factors and fluctuating levels of melatonin

“…Folate and B6 vitamin are supposed to boost the formation of serotonin from TRP as coenzymes. Zinc and magnesium, instead, are supposed to enhance the formation of melatonin from serotonin by binding to AANAT enzyme, thus activating it and increasing the affinity of serotonin for binding to AANAT (74, 75). In humans, the role of these vitamins and minerals is less well studied in this connection.…”

Involvement of Calcium in Pineal Gland Function

“…At a molecular level, calcium ions have been shown to inhibit the activity of the pineal (HIOMT) (28,29) and to enhance the activity of NAT (28,30), such inferences being made on the basis of changes to the double-reciprocal plots of the Michaelis-Menten kinetic function (Figs. 5 and 6) (29,30).…”

“…Calcium is apparently an uncompetitive inhibitor of HIOMT, binding to the enzyme/substrate complex, which leads to formation of a catalytically inactive complex (29). Activation of NAT by calcium results from binding of the cation to the enzyme; this leads to an increased affinity of serotonin for binding to the NAT molecule, resulting in enhanced catalytic activity (30). This mechanistic scheme for calcium activation of NAT would imply that the cation bridge must be in place before catalysis can occur; therefore, it is assumed that the enzyme is dependent on the presence of calcium for the production of melatonin (30).…”

“…Activation of NAT by calcium results from binding of the cation to the enzyme; this leads to an increased affinity of serotonin for binding to the NAT molecule, resulting in enhanced catalytic activity (30). This mechanistic scheme for calcium activation of NAT would imply that the cation bridge must be in place before catalysis can occur; therefore, it is assumed that the enzyme is dependent on the presence of calcium for the production of melatonin (30). To what extent the intervention of calcium at a molecular level influences pineal function is not known, and progress is hampered by difficulties encountered in purification of NAT for detailed studies.…”

Decreased Melatonin Biosynthesis, Calcium Flux, Pineal Gland Calcification and Aging: A Hypothetical Framework

“…On a molecular level, calcium ions apparently have inhibitory and activating properties on var ious enzymes and in such a way do not only regulate but balance catalytic activity [43][44][45]. Binding of calcium to N-acetyltransferase results in activation of the enzyme and, subse quently, increased affinity for serotonin [46]. That cation bridge must be in place before catalysis can occur and is actually considered rate limiting for the synthesis of melato nin.…”