Cystathionine synthase in rat brain: Regional and time-of-day differences and their metabolic implications

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Abstract
In relation to concern for probable interrelations of cystathionine synthase (CS, EC 4.2.1.21) and methionine, one‐carbon, and 5‐hydroxytryptamine metabolism, we have investigated tissue and time‐of‐day differences in CS activity in the laboratory rat under standardized conditions. Liver, kidney, and pancreas had highest CS activities; nine regions of the CNS had mean activities ranging from 4.5% (lumbosacral cord) to 24.5% (hypothalamus) of mean hepatic activity; pituitary and adrenal glands lacked detectable CS activity. Although significantly lower CS activity occurred in liver (↓ 9%, P < 0.050) and kidney (↓ 13%, P < 0.025) during the interval two hours before to two hours after the daily onset of darkness, no significant changes were found in hypothalamus, cerebellum, or medulla oblongata. Regional CNS differences in CS activity appeared to be without correlation in relation to published data on relative contents of 5‐hydroxytryptamine, tryptophan 5‐hydroxylase (EC 1.14.16.4), 5,10‐methylene reductase N5‐methyltetrahydrofolate‐NAD‐oxidoreductase, (EC 1.1.1.78), or 5‐methyltetrahydrofolate. Therefore, among CNS regions examined, a critical deficiency in ability to metabolize a homocysteine load is considered to be unlikely under normal conditions.