[0061] The lipoic acid compounds which can be used in the invention as the competing substrate are those which are reduced in the presence of diaphorase and NAD(P)H and include for example the following: the esters, such as alkyl, e.g. lower alkyl esters (from 1 to 6 carbon atoms), aryl esters, (including alkaryl esters), e.g. having 6-10 carbon atoms including benzyl and phenyl esters, amides lik