Effect of heavy metals on the uptake of [ 3H]- l-histidine by the polychaete Nereis succinea

Abstract

Integumentary uptake of [ 3H]- l-histidine by Nereis succinea was measured in the presence and absence of selected heavy metals and the amino acid l-leucine in 60% artificial seawater (ASW). The time course of 10 μM [ 3H]- l-histidine uptake into worms over a 60 min incubation was approximately doubled in the presence of 0.5 μM zinc and when calcium in the incubation medium was reduced from 6 mM to 5 μM the stimulatory effect of zinc on amino acid accumulation was reduced and uptake under the latter conditions was approximately half that of the control. Zinc stimulation of [ 3H]- l-histidine influx was a hyperbolic function of zinc concentration over the range 0 to 50 μM metal and displayed an apparent activation or affinity constant of 385±127 nM Zn 2+. The hyperbolic stimulatory effect of 1 μM Zn 2+ on the time course of 10 μM [ 3H]- l-histidine uptake was abolished in the presence of 25 μM l-leucine, suggesting that this amino acid shared the same transport system as [ 3H]- l-histidine and acted as a potential competitive inhibitor. Influx of [ 3H]- l-histidine was a hyperbolic function of external amino acid concentration and displayed an apparent affinity constant ( K m ) of 23.71±5.02 μM and an apparent maximal velocity ( J max) of 4701±449 pmol/g dry wt.×15 min. Addition of 0.5 μM zinc resulted in a four-fold increase in J max and a doubling of K m , suggesting the effect of the metal was mostly on the rate of amino acid transport. [ 3H]- l-histidine influx was mildly stimulated by Fe 2+ (0.5 μM), but was unaffected by either Ag + or Al 3+ (both at 0.5 μM). These results suggest that [ 3H]- l-histidine uptake into worm integument may take place by the classical Na +-independent l-transport system shared by l-leucine and regulated by exogenous calcium and other divalent metal concentrations.