Glycine Uptake by Cultured Human Y79 Retinoblastoma Cells: Effect of Changes in Phospholipid Fatty Acid Unsaturation

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Abstract
Glycine uptake was investigated in cultured Y79 retinoblastoma cells containing different degrees of phospholipid fatty acid unsaturation. The modifications were produced by growing the retinoblastoma cells in medium supplemented with various unsaturated fatty acids. Glycine was taken up by the retinoblastoma cells through two kinetically distinguishable processes. The high-affinity system is totally dependent upon extracellular Na+ and partially dependent upon Ca2+. Of the glycine taken up by retinoblastoma cells, 85–90% remains as free intracellular glycine and less than 3% is incorporated into cellular protein. When the cells are grown in a medium containing 10% fetal bovine serum as the only source of fatty acids, the phospholipids contained 23% polyunsaturated fatty acids. Under these conditions the high-affinity system has a K′m of 34.2 ± 3.7 μM and a V′max of 91.2 ± 16.2 pmol min−1 mg protein−1. The low-affinity system has a K′m of 2.7 ± 0.4 mM and a V′max of 4.1 ± 0.5 nmol min−1 mg protein−1. When the polyunsaturated fatty acid content of the phospholipids was increased by supplementing the medium with linolenic or docosahexaenoic acids (n-3 polyunsaturates) or linoleic or arachidonic acids (n-6 polyunsaturates), the K′m and V′max of the high-affinity glycine uptake system were increased three- to fourfold. By contrast, supplementing the medium with oleic acid, an n-9 monounsaturate, did not significantly alter the K′m or K′maxfor glycine uptake. The results with this model system suggest that one of the effects of the high polyunsaturated fatty acid content normally present in neural cell membranes may be a modulation of the high-affinity transport system so that it functions more efficiently in regulating glycine uptake.