GENETIC-VARIATION IN THE TEMPERATURE-DEPENDENCE OF LIVER MICROSOMAL CYP2E1 ACTIVITY, WITHIN AND BETWEEN SPECIES OF THE VIVIPAROUS FISH POECILIOPSIS

The temperature dependence of liver microsomal CYP2E1 (cytochrome P450 2E1) activity was examined in selected genotypes of the viviparous fi sh Poeciliopsis. Activity of this enzyme, as a function of incubation temperature, was determined by measuring 6-OH-chlorzoxazone formation from chlorzoxazone, a specific CYP2E1 substrate. Chlorzoxazone-6-hydro xylase activity was examined among five species of Poeciliopsis, as we ll as among nine genotypes within a species, P. monacha. Among Poecili opsis genotypes, P. monacha contained the greatest activity, 9.5 +/- 1 .5 U (where 1 U is defined as 1 mu g 6-OH-chlorzoxazone produced/min/m g protein; n = 6) with a temperature optimum (T-o) of 25 degrees C. Th e lowest activity was in P. occidentalis, 0.65 +/- 0.11 U, with a T-o of 27 to 28 degrees C; P. prolifica, P. fasciata, P. lucida, and P. vi riosa had intermediate levels of activities, 1.1 to 5.5 U, and T-o fro m 25 to 31 degrees C. To determine if metabolic differences exist with in species, enzyme activity was examined from nine genotypes of P. mon acha by comparing expression among P. monacha-lucida hybrids. These hy brids were given identical paternal genomes of lucida but retained the original maternal monacha genomes sampled from wild gene pools. The g reatest activity was found in genotype T70-3 P. Cw, 3.6 +/- 0.1 U, at a T-o of 29 degrees C, and the lowest was in genotype SV73-7s, 0.40 +/ - 0.12 U, at a T-o of 27 degrees C. The other naturally occurring geno types, M65-24, M65-26, SV-73-7v, as well as the laboratory-produced sy nthetic hybrids, Syn4 and Syn5, had intermediate activities, 0.73 +/- 0.38 to 2.1 +/- 0.69 U, and T-o 25 to 27 degrees C. No hybrid had acti vity levels as high as the maternal parent, P. monacha, and only one h ad a T-o as low as either parent. Apparently the genes involved in xen obiotic activity vary widely among the closely related species of Poec iliopsis but also within species, suggesting that these phenotypes can be acted upon by natural selection.