Cystathionine β-Synthase Deficiency: A Qualitative Abnormality of the Deficient Enzyme Modified by Vitamin B6 Therapy

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Abstract
Summary: The thermostability of cystathionine synthase and the effect of pyridoxal phosphate (PLP) on this thermostability were investigated in extracts of normal human liver and in extracts of liver, both before and during pyridoxine (vitamin B6) therapy, from members of a family with three clinically and biochemically typical, B6-responsive, synthase-deficient sibs. Incubation of crude extracts of normal liver at 55° (preincubation) for 3–4 min before assay consistently resulted in a more than 2-fold increase in specific activity (activation) of cystathionine synthase (Fig. 1). With periods of preincubation longer than 4 min, thermal inactivation occurred. When PLP was added to the preincubation mixture, slightly more activation occurred in the first 3–4 min, and there was no observable loss of activity for an additional 25 min. The activation phenomenon was not observed in extracts of liver which had been obtained from three synthase-deficient sibs before therapy with vitamin B6 (Index of activation, Table 1). When extracts of liver obtained during vitamin B6 therapy were studied, however, significant activation was observed. Synthase activity in extracts of liver from the patients' parents, obligate heterozygotes for synthase deficiency, and from a potentially heterozygous sister demonstrated activation similar to that found in control liver extracts. With periods of preincubation longer than 5 min, the inactivation of synthase in liver extracts from patients receiving pyridoxine-HCI occurred at the same rate as in liver extracts from heterozygotes and from normal subjects (Index of inactivation, Table 1). PLP completely prevented heat inactivation of enzyme from normal liver. Speculation: Activation of cystathionine synthase is an expression of the ability of the normal enzyme to undergo heat-induced conformational changes. The failure to activate the abnormal enzyme suggests it has an altered molecular structure.