chemical reaction, they possess markedly dif- 

 ferent physical and chemical properties. In the 

 light of this evidence, it seems reasonable to 

 conclude that isozymes are groups of molecules 

 of common origin that have become differentiated 

 to meet highly specific requirements within the 

 cell. The specialization of the individual iso- 

 zymes indicates that they may be located indif- 

 ferent places within the cell, or concentrated 

 in different kinds of cells and tissues. Evidence 

 in this regard has been brought forth by several 

 investigators (23, 24, 25). 



An insight into the physiological role of 

 individual isozymes has been provided through 

 determinations of their optimal substrate con- 

 centrations. The earliest investigation, by Plage- 

 mann et al., revealed that the optimal pyruvate 

 concentration for human LDH- 1 is considerably 

 lower than that for human LDH- 5 (26). This has 

 now also been established for LDH- 1 and LDH- 5 

 from other vertebrates (27). For example, in a 

 series of experiments carried out in our labora- 

 tory (summarized in Fig. 15), it was observed 

 that the optimal pyruvate concentration of horse 

 LDH-1 is distinctly lower than that of horse 

 LDH- 5. In the case of the fluke whose tissues 

 reveal only a single band of LDH activity as 

 analyzed by starch gel electrophoresis, both 

 heart and skeletal muscle LDH appear to have 

 identical substrate optima. This and other data 

 (28) indicate that fluke heart LDH is identical 

 to fluke skeletal muscle LDH. Since the pyru- 

 vate optimum of fluke LDH is similar to that of 

 horse LDH- 5, it seems reasonable to assume 

 that other properties of fluke LDH would be 

 similar to vertebrate LDH- 5 and that fluke LDH 

 is, in effect, an LDH- 5. 



These observations are significant in that 

 LDH- 5 is found mainly in tissues, such as 

 skeletal muscle, which are subject to periods 

 of relative anaerobiosis and consequently are 

 subjected to relatively high concentrations of 

 pyruvate and lactate due to an increased func- 

 tioning of the glycolytic pathway and decreased 

 functioning of the tricarboxylic acid cycle. On 

 the other hand, LDH-1 is found mainly in well 

 oxygenated tissues with a high aerobic metabo- 

 lism such as heart and brain in which high con- 

 centrations of pyruvate and lactate are not 

 encountered. 



An interpretation of this data involves the 

 effect of high concentrations of lactate on muscle 

 tissue. As is well known, during violent€xercise, 

 lactate can accumulate in skeletal muscle until 

 the muscle is paralyzed. Obviously, this cannot 

 be allowed to occur in heart muscle. The inhibi- 



tion of heart muscle LDH at relatively low con- 

 centrations of pyruvate, then, acts as a check 

 valve which functions to shunt pyruvate into the 



( + ) 



Xcv %. ^xv X^ %. 'V «> . 



Fig. 14. 



Interspecific hybridization of chicken LDH. The hybridi- 

 zations were performed with LDH obtained from pooled 

 tissues of each of the organisms. Note the complexity of 

 these hybrid patterns as compared to those illustrated in 

 Fig. 12.(FromMarkert, in Ideas in Modem Biology, 1965; 

 reproduced with permission of the National Academy of 

 Sciences.) 



87 



