Oxidation of [U-14C]Succinic Semialdehyde in Cultured Human Lymphoblasts: Measurement of Residual Succinic Semialdehyde Dehydrogenase Activity in 11 Patients with 4-Hydroxybutyric Aciduria

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Abstract
The oxidation of [U-14C]succinic semialdehyde to 14CO2 has been investigated in cultured lymphoblasts to develop a whole cell assay for succinic semialdehyde dehydrogenase. We have previously demonstrated deficiency of this enzyme in extracts of white cells derived from 13 patients with 4-hydroxybutyric aciduria. Major goals were the demonstration of greater residual succinic semialdehyde dehydrogenase activity in patient cell lines and the better representation of physiology in vivo. In 18 control lymphoblast lines, the conversion of [U-14C]succinic semialdehyde to 14CO2 was 1579 ± 310 dpm. The mean value in lymphoblasts derived from 11 patients with deficiency of succinic semialdehyde dehydrogenase was 112 ± 36 dpm approximating 7% of the mean control value. Analysis of organic acids produced from [U-14C]succinic semialdehyde in control lymphoblasts indicated that 14CO2 emanated from the tricarboxylic acid cycle; the major metabolic products were succinic and lactic acids. In the presence of 5mM malonic and 2-propylpentanoic (valproic) acids, 14CO2 production in a control lymphoblast fine was decreased by 68 and 45%, respectively. The whole cell assay is less laborious than our previously described assay employing cell extracts, and the general trend was the demonstration of higher residual levels of activity for lymphoblasts derived from patients.