Branched-chain aminotransferase deficiency in Chinese hamster cells complemented by two independent genes on human chromosomes 12 and 19

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Abstract
Branched-chain aminotransferase (BCT) catalyzes the reversible transamination of the branched-chain α-keto acids to the branched-chainl-amino acids. Since branched-chainl-amino acids (l-isoleucine,l-leucine, andl-valine) are essential for cell growth, cells which lack BCT were unable to proliferate in media containing α-keto acids in place of the correspondingl-amino acids. CHW-1102, a Chinese hamster cell line, lacks BCT and does not grow in α-keto acid media. Somatic cell hybrids were made by the fusion of CHW-1102 (HPRT−) with several human cell lines and isolated on HAT medium. Growth assays of hybrid clones on α-keto acid selection media independent of the HAT selection medium indicated two cell hybrid phenotypes: either (1) the hybrid clone, like the parental CHW-1102, could not utilize α-keto acid media, or (2) the hybrid could proliferate on all three α-keto acid media. The ability of hybrid cells to proliferate on α-keto acid media correlated with the presence of either of two human genes which independently complemented the Chinese hamster deficiency. Two human genes, BCT1 assigned to chromosome 12 and BCT2 assigned to chromosome 19, were demonstrated to code for the expression of two molecular forms of BCT.