PURIFICATION AND AMINO-TERMINAL AND CARBOXYL-TERMINAL AMINO-ACID-SEQUENCES OF ALANINE-GLYOXYLATE TRANSAMINASE-1 FROM HUMAN LIVER

In order to confirm the amino acid sequence predicted from the nucleotide sequence of cDNA and also to elucidate the intracellular localization and molecular evolution, human liver alanine-glyoxylate transaminase 1 (AGT1) was purified and subjected to partial amino acid sequence determination, with special attention to posttranslational modification. The enzyme was purified to homogeneity from the 10,000Xg supernatant of human liver homogenate. The purified enzyme showed only a single protein band at about 43 kDa on SDS-PAGE, indicating that it is a homodimer of two identical subunits, because the native enzyme has a molecular mass of about 80 kDa. Both the amino- and carboxyl-terminal peptides of the enzyme were isolated from a cyanogen bromide digest of the S-carboxymethylated protein and subjected to amino acid sequence determination, The alpha-amino group of the amino-terminal peptide was shown to be blocked by an acetyl group. The carboxyl-terminal sequence contained a putative N-glycosylation sequence (-Asn-Ala-Thr-), the only one present in the whole molecule, but this sequence was normally determined, indicating that the enzyme is not N-glycosylated. Purdue et al. [J. Cell Biol. 111, 2341-2351 (1990)] have reported that Pro-11, Gly-170, and Ile-340 in normal human AGT1 were replaced by Leu, Arg, and Met, respectively, in a patient with primary hyperoxaluria type 1. We confirmed that residue-11 was Pro. Both the amino- and carboxyl-terminal sequences of the enzyme showed extensive similarity with those of rat liver mitochondrial serine-pyruvate aminotransferase and the small chain of hydrogenase from a thermophilic unicellular cyanobacterium, Synechococcus PCC 6716. A CySH residue in the carboxyl-terminal region of rat liver mitochondrial serine-pyruvate aminotransferase was also found in human liver alanine-glyoxylate transaminase 1, implying that this CySH residue may be involved in the function of the two enzymes.