(+) 



LDH 

 I 



LDH 



(-) 



0= HYBRrO 

 JSOZYMES 



s \ % %^ % % \ \ \ 



\ \ \ V \ 



'H^. 



\ 



Fig. 12. 



Interspecific hybridization of horse LDH. The Isozymes 

 of native LDH are designated by the numbers 1, 2, 3, 4, 

 5, while the hybrid isozymes are designated 0. Fish re- 

 fers to the herring, -l/osa aestivalis, and salamander to the 

 newt, Diemictytus viridescens. (From Markert, in The 

 Harvey Lectures, Series 59, 187, 1965; reproduced with 

 permission of Academic Press.) 



Fig. 13. 



Interspecific hybridization of horse LDH. Chicken refers 

 to White Leghorn chicken heart LDH and snake to the 

 LDH of pooled tissues of the diamondback rattlesnake 

 (Crotalus adamanteus). The numbers along the ordinate 

 designate the isozymes of horse LDH. 



interspecific hybrid molecules have catalytic 

 properties analogous to those of the intraspecif ic 

 hybrid molecules. However, as the evolutionary 

 distance between species increases, the 

 enzymatic activity of the hybridized preparation 

 decreases. This loss in activity may be due to 

 the formation of completely or partially inactive 

 polymers. 



Mammalian LDH-1 (the tetramer composed 

 of B subunits) will combine with LDH- 5 (the 

 tetramer composed of A subunits) from any of 

 the other six vertebrate classes to form at 

 least three hybrid molecules. In an identical 

 hybridization in which there are two kinds of 

 B subunits with respect to charge, theoretically, 

 fifteen isozymes can be formed. If , in addition, 

 two differently charged A subunits are involved, 



then thirty-five different isozymes should be 

 formed. However, the resolution of thirty-five 

 isozymes may exceed the capabilities of the 

 starch-gel electrophoretic system. In any event, 

 as many as twenty-five distinct bands have been 

 counted on the zymograms of certain hybridiza- 

 tions such as between chicken and horse. 



The occurrence of isozymes of LDH in 

 nearly all vertebrates which have been ex- 

 amined strongly suggests that, for certain en- 

 zymes, multiplicity of form is evolutionarily 

 advantageous and does not represent simple 

 heterogeneity of no biological value. The impli- 

 cation is that the individual isozymes subserve 

 a specialized role in the economy of the orga- 

 nism. This is supported by the fact that, although 

 all isozymes of LDH catalyze a characteristic 



86 



