Characteristics of leucine transport by isolated hepatocytes of antarctic fish at low temperatures

Hepatocytes prepared by collagenase perfusion from Antarctic nototheniid fish of genus Trematomus are active in uptake of ¹⁴C]leucine at 0, 5, and 10°C. The system is saturable with apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ about 1.0 mM. Isoleucine and phenylalanine were major competitors, valine was about one-half as effective, while alanine, glycine and histidine had no effect. Temperature dependency of rates in the 0–10°C range yielded $\text{E}{}_{\mathrm{<mtext>a</mtext>}}\text{= 65}\text{kJ/mol}$ ($\text{Q}{}_{10}\text{= 2.7}$). The average first-order rate constant at 0°C was 0.1 min^?1, one-third the value of 0.3 min^?1 estimated for clearance of ¹⁴C]leucine by liver of these species in vivo. Affinity and specificity agreed well with in vivo data on liver clearance of leucine, both in Antarctic fish at 0°C and in temperate fish acclimated to 10°C and 20°C. The results indicate similar modifications of leucine transport associated with evolutionary cold adaptation and seasonal acclimation in fish.