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Abstract
A study has been made of the accumulation of the amino acid C14-labeled alpha amino isohutyric acid (α-AIB) in rabbit lenses cultured under varying conditions. In a medium containing no other amino acids, this compound accumulates at approximately a linear rate for the maximum period studied (48 hours), at which time the ratio of concentration in lens ivater to concentration present initially in the medium, is 22. Addition of nonlabeled α-AIB to culture medium saturates the transport system, the uptake of the tracer amino acid decreasing asymptotically with the total concentration of amino acid present. The apparent Michaelis-Menten constant km is 2.5 mmoles per liter and the maximum velocity is 0.25 µmoles per lens per hour. Neutral, but not basic or acidic, amino acids inhibit the transport of labeled α-AIB into the lens in varying degree. The gamma isomer of α-AIB does not inhibit transport and the D forms of neutral amino acids are less effective inhibitors than the L forms. At least three separate systems, perhaps carriers or carrier sites, are involved in transporting amino acids in the lens. Accumidation of α-AIB requires energy, about three quarters of which can be supplied anaerobically through consumption of glucose, is highly dependent upon temperature, and can be inhibited by various metabolic poisons. Mechanisms responsible for active transport of amino acids are associated with the capsule and epithelium. Binding is not responsible for the concentration gradient between amino acids in the lens and its environment. The lens acts like a "pump-leak system," the steady state concentration being determined by a balance between accumulation of amino acids through active transport and loss by diffusion. The system responsible for active transport of neutral amino acids in the lens in vivo appears to have a large reserve capacity for moving these compounds against appreciable concentration gradients.