Amino acid transport and crystallin synthesis in the bovine lens

Bovine lenses were incubated in a defined, bicarbonate-free culture medium (EMEM) and the kinetics of amino acid uptake and protein synthesis investigated. The kinetics were interpreted in terms of a simple multi-compartment model. [¹⁴C]tyrosine was found to be totally exchangeable in the incubated lens and the rate constant for the exponential increase in activity was 0·0175 hr^?1. The rate of influx was markedly reduced by incubating in the presence of ouabain (10^?5m), which also caused a concomitant disturbance of the normal sodium and potassium distributions.The soluble proteins from the ineubated lenses were fractionated on Sephadex G-200 and the rate of incorporation into the crystallins was shown to fall into two classes. The rate of synthesis of α and β_L crystallin was relatively rapid (rate constants approximately equal to 0·004 hr^?1), while the synthesis rates of β_H and $\text{β}{}_{\mathrm{<mtext>S</mtext>}}\text{γ}$ were both much slower (0·001 hr^?1).The efflux kinetics of [¹⁴C]tyrosine were determined and the rate of decrease of the free amino acid pool was identical to the rate of increase determined from an influx experiment. Hence the lenses are in a steady state with respect to free tyrosine throughout the incubation period (up to 160 hr). All classes of proteins continued to be synthesized during efflux experiments and there was no evidence for a breakdown of α or β_L crystallin during the time-course of these experiments.Ouabain slowed the rate of loss of tyrosine from the free amino acid pool, and this was interpreted in terms of an ouabain-induced decrease in synthesis rate rather than as a decrease in efflux rate from the lens. There was in fact a very marked decrease in the incorporation of [¹⁴C]tyrosine into the α and β_L crystallins on exposure to ouabain, and this decrease was apparent before and change in activity in the amino acid pool.