Zur Theorie der Cocarboxylasewirkung, II. Ableitung und Begründung eines Zweizentrenmechanismus der Thiaminpyrophosphatwirkung aus Modellversuchen und enzymatischen Messungen

Abstract

Derivatives of thiamine, obtained by substitution of elimination of the amino group, act as model catalysts in the non-enzymatic formation of acyloins (Mizuhara test); their mode of action is essentially different from that of their pyrophosphate esters in the enzymatic reaction with apo-pyruvate decarboxylase. The "active" bound in 2-[1-hydroxyethyl]-thiazolium compounds can be cleaved by nucleophilic agents, e. g., 4-aminopyrimidines, with the release of acetaldehyde. Thiamine pyrophosphate and its 4[image]-hydroxy-4[image]deamino derivative compete for the same site of binding to the apoenzyme. When the enzymatically inactive system 4[image]-hydroxy-4[image]-desaminothiamine pyrophosphate/Mg/apodecarboxylase is incubated with [2-14C] or [1-14C]pyruvate, an analogue of "active acetaldehyde" is formed. This shows that the absent amino group is necessary for the release of the aldehyde molecule. The results of varied model experiments and those from enzymatic studies were reconciled by an intramolecular SN-mechanism (double centre mechanism) between the C-2-position of the thiazolium ring and the amino group at position 4[image] of the coenzyme.