INHIBITION OF GLUCOSEPHOSPHATE ISOMERASE ALLOZYMES OF THE MOSQUITOFISH, GAMBUSIA-HOLBROOKI, BY MERCURY

Frequencies of allozyme genotypes are being used as population-level i ndicators of environmental heavy-metal contamination. A genotype of gl ucose phosphate isomerase, Gpi-2(38/38), of mosquitofish (Gambusia hol brooki) has been identified as ''mercury-sensitive'' in an acute toxic ity assay. Partially purified preparations of GPI-2 38/38 and GPI-2 10 0/100 were assayed to determine differences in maximum gluconeogenic r eaction velocity at seven mercury (added as HgCl2) concentrations, 15 to 960 nM Hg. Log-Probit analysis of the inhibition curves indicated t hat the log(IC50) (log10 of the Hg concentration causing a 50% reducti on in reaction velocity) for GPI-2 100/100 was significantly lower tha n that for GPI-2 38/38, even though GPI-2 100/100 initial uninhibited reaction velocity was greater than that of GPI-2 38/38. Although the m echanism of inhibition was not experimentally determined, under the as sumption of noncompetitive interaction between Hg and GPI-2, the inhib itor dissociation constants (95% asymptotic C.I.) for GPI-2 100/100 an d GPI-2 38/38 were estimated from the log (IC50) as 204 nM Hg (155-269 nM Hg) and 479 nM Hg (363-617 nM Hg), respectively. These results sug gested that Hg susceptibility related to the Gpi-2(38/38) genotype in acute toxicity assays was likely not due to enhanced Hg inhibition of GPI-2 38/38.