Transport of Methionine in Sea‐Urchin Sperm by a Neutral Amino‐Acid Carrier

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Abstract
A carrier-mediated transport for L-methionine and other neutral amino acids exists in sperm of the sea urchin L. pictus. The initial rate of L-methionine entry is a Michaelis-Menten function of the methionine concentration in the external medium. The maximum velocity is low [V = 250 pmol h-1 (109 sperm)-1 at 22.degree. C] and the affinity is high (Km = 6-10 .mu.M). The initial rate of transport under steady-state exchange conditions is also a Michaelis-Menten function of the external concentration of methionine. The Km determined by this method is .apprx. 14 .mu.M. Neutral amino acids compete with L-methionine transport as shown by initial velocity measurements. L-methionione transport probably is a carrier-mediated process. The temperature dependence of the process is .apprx. 84 kJ (20 kcal) mol-1 K-1, which is not compatible with a simple diffusion mechanism, but in the range of values usually found for a mediated transport. The transport is largely Na+-dependent and does not depend on Ca2+, K+ or H+ gradients. It is only partially sensitive to KCN, showing it is mainly independent of oxidative phosphorylation. The steady-state internal methionine concentration is not a linear function of the external amino acid concentration. An exit by diffusion competes with a carrier-mediated concentrative transport in a cellular compartment. This mediated transport is compared to those of higher animal cells.