Enzymatic urea adaptation: lactate and malate dehydrogenase in elasmobranchs.

Lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) electrophoretic tissue patterns of two different orders of Elasmobranchii: Carchariniformes (Galeus melanostomus and Prionace glauca) and Squaliformes (Etmopterus spinax and Scymnorinus licha) were studied. The number of loci expressed for these enzymes was the same of other elasmobranch species. Differences in tissue distribution were noted in LDH from G. melanostomus due to the presence of an additional heterotetramer in the eye tissue. There were also differences in MDH. In fact, all the tissues of E. spinax and G. melanostomus showed two mitochondrial bands. Major differences were noted in the number of isozymes detected in the four compared elasmobranchs. The highest polymorphism was observed in E. spinax and G. melanostomus, two species that live in changeable environmental conditions. The resistance of isozymes after urea treatment was examined; the resulting patterns showed a quite good resistance of the enzymes, higher for LDH than MDH, also at urea concentration much greater than physiological one. These results indicated that the total isozyme resistance can be considered higher in urea accumulators (such as elasmobranchs) than in the non-accumulators (such as teleosts).