Pyruvate stereospecific reduction

On the other hand a stereospecific reaction is one in which a particular stereoisomeric form of the reactant reacts to give a stereoisomeric form of the product. The enzymatic reduction of pyruvic acid is not stereospecific because the reactant can not existin stereoisomeric form. Therefore, all reactions that are stereospecific are necessarily stereoselective but not all stereoselective reactions are stereospecific. 

In marked contrast, nature s reducing agent, reduced nicotinamide adenine dinucleotide (NADH), delivers hydride in a stereospecific manner because it is a cofactor in an enzyme-catalysed reaction. For example, reduction of pyruvic acid to lactic acid in vertebrate muscle occurs via attack of hydride to produce just one enantiomer, namely (5)-lactic acid. 

But the results provided an exciting bonus. The reactions proved to be stereospecific. NAD+ that has been reduced with dideuterioethanol has both a hydrogen atom and a deuterium atom in the 4-position of the pyridine ring, but reductions of pyruvate or of acetaldehyde with NAD(D) prepared enzymically in this way quantitatively transfer deuterium, and leave the hydrogen atom in place. The 1954 diagram showing the stereospecific transfer of hydrogen from the 4-position of NADH is presented in (5). 

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