Publisher Summary This chapter presents an overview of the early research conducted on isoenzymes. By 1959, Markert and Moller had demonstrated the presence of isoenzymes in various tissues using an overlay method after zone electrophoresis in starch gels. In 1960, Elliot Vesell, in the Rockefeller laboratory of Alexander Bearn, conducted research on separating the LDH isoenzymes electrophoretically by starch gel electrophoresis and then directly visualizing the enzyme in the gel. Kaplan established his laboratory at Brandeis University in 1950s and worked on enzymes that required nicotinamide adenine dinucleotide (NAD) as a co-factor. The Kaplan Laboratory actively synthesized analogues of NAD, and found that the ratio of enzymatic activity with NAD and its various analogues was a very sensitive measure of the differences of various dehydrogenases (especially LDH) in different species and in different organs. This chapter discusses about the discovery that demonstrated that lophurae LDH had an exceptionally high affinity for the acetylpyridine analogue of NAD. The GOGAT reaction is discussed. This reaction produces two molecules of glutamate that can yield, via GDH, two NADPH that can in turn serve as an electron source for the antioxidative enzymes glutathione reductase (GR) and thioredoxin reductase.