Tetrahydrothiamin pyrophosphate, an analogue of thiamin pyrophosphate (TPP) in which the thiazolium ring has been reduced to a thiazolidine ring, was prepared by borohydride reduction of TPP. It consists of four stereoisomers, comprising two diastereomers each of which is a racemic mixture, generated by the introduction of two chiral centers on the thiazolidine ring. The major and minor diastereomers were separated and inferred to be of the cis and trans configurations, respectively, from a study of the nuclear Overhauser effects in the 'H NMR spectrum of the simpler tetrahydrothiamin. Tetrahydro-TPP behaves as a mixture of potent inhibitors of the pyruvate decarboxylase (El) component of the pyruvate dehydrogenase complex from Escherichia coli. The site of binding is probably the TPP-binding site on El, and the Kd for each of the four stereoisomers was estimated. The cis isomers of tetrahydro-TPP bind more tightly than does TPP, whereas the trans isomers appear to bind with about the same Kd as TPP. Sodium borohydride caused a rapid inhibition of El activity in the presence of TPP, believed to be due to reaction of borohydride with enzyme-bound TPP. The experiments are consistent with an enhancement of the reactivity of the thiazole ring of TPP when bound to the catalytic site of El, which could be due to polarization of the +N=C bond near a hydrophobic or positively charged region of the protein. A spontaneous reactivation occurred after the initial inhibition by borohydride, attributable to a weakly binding inhibitor, not tetrahydro-TPP, being formed at the catalytic site.
Department of Chemistry
University of Cambridge