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

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 microM [3H]-L-histidine uptake into worms over a 60 min incubation was approximately doubled in the presence of 0.5 microM zinc and when calcium in the incubation medium was reduced from 6 mM to 5 microM 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 microM metal and displayed an apparent activation or affinity constant of 385+/-127 nM Zn(2+). The hyperbolic stimulatory effect of 1 microM Zn(2+) on the time course of 10 microM [3H]-L-histidine uptake was abolished in the presence of 25 microM 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 (Km) of 23.71+/-5.02 microM and an apparent aximal velocity (J(max)) of 4701+/-449 pmol/g dry wt.x15 min. Addition of 0.5 microM 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 microM), but was unaffected by either Ag(+) or Al(3+) (both at 0.5 microM). 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.