Human lymphocyte methionine adenosyltransferase (HuLy MAT) consists of heterologous subunits and . The cDNA sequence of the subunit of HuLy MAT from Jurkat leukemic T cells was identical to that of the human kidney subunit and highly homologous to the sequence of the extrahepatic MAT from other sources. The 3`-untranslated sequence was found to be highly conserved, suggesting that it may be important in regulating the expression of MAT. The extrahepatic subunit of MAT was found to be expressed also in human liver, and no differences were found in the sequence of the subunit from normal and malignant T cells. The sequence of two unspliced introns found in the cDNA clones from the Jurkat library enabled us to isolate genomic clones harboring the human extrahepatic subunit gene and to localize it to the centromere on chromosome arm 2p, an area that corresponds to band 2p11.2.

Expression of the subunit cDNA in Escherichia coli yielded two peptides with the immunoreactivity and mobilities of authentic /` subunits from HuLy. The K of the recombinant subunit was 80 µM, which is 20-fold higher than found for the (`) holoenzyme purified from leukemic lymphocytes and 4-10-fold higher than found for the normal lymphocyte enzyme. The data suggest that the /` subunits mediate the enzyme catalytic activity and that the subunit may be a regulatory subunit of extrahepatic MAT.

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