Stable carbon isotope discrimination by human 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

The aim of this study was to investigate the possible existence and magnitude of stable carbon isotope discrimination by human 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). The catalytic portion of HMGR was expressed and purified. The reaction product mevalonate was lactonized and extracted from the reaction mixture by a solid-phase extraction protocol. Stable carbon isotope ratios of mevalonolactone (MVL) were analyzed by gas chromatography-combustion-isotope ratio mass spectrometry. An average fractionation factor (12)k/(13)k of 1.0031 +/- 0.0004 for all carbon atoms contained in MVL was estimated by the method of internal competition. The value was calculated by nonlinear curve fitting, where the ratio (13)C/(12)C of MVL was plotted versus the fraction of reaction.