Some characteristics of amino acid transport in frog ovarian oocytes

Frog ovarian oocytes have been incubated in physiological saline solutions containing l-amino acids. Phenylalanine and tryptophan can be accumulated in the cells to internal concentrations many times that of the medium.The amino acid transport for phenylalanine is competitively inhibited to a small extent by l-leucine and to a greater extent by l-tyrosine. One animal was used in late spring whose phenylalanine transport was normally inhibited by l-tyrosine but uniquely not inhibited by l-leucine. This observation was interpreted to mean that the steric specificity of the site can change. The change could be due to maturational development or perhaps a genetic variation.Once taken up into the cell, free phenylalanine is almost quantitatively lost to the medium over a period of time after superficial damage to the cell surface. Internal binding of the amino acids is thus either extremely labile or non-existent.The phenylalanine mediation shows Michaelis-Menten kinetics in its dependency of rate of uptake on the extracellular concentration of amino acids.Although cyanide was found in other studies on oocytes to stop protein synthesis immediately and completely, in these experiments it slowed down the initial rate of amino acid entry relatively little. The per cent of inhibition over the controls becomes progressively more marked during a period of 6 hr. Thus in short term incubations, amino acid uptake can be completely uncoupled from amino acid incorporation but long term accumulation against a concentration gradient requires respiratory metabolism.