Abstract
Mouse erythrocyte guanine deaminase has been purified to homogeneity. The native enzyme was dimeric, being comprised of two identical subunits of approximately 50,000 Da. The protein sequence was obtained from five cyanogen bromide cleavage products giving sequences ranging from 12 to 25 amino acids in length and corresponding to 99 residues. Basic Local Alignment Search Tool (BLAST) analysis of expressed sequence databases enabled the retrieval of a human expressed sequence tag cDNA clone highly homologous to one of the mouse peptide sequences. The presumed coding region of this clone was used to screen a human kidney cDNA library and secondarily to polymerase chain reaction-amplify the full-length coding sequence of the human brain cDNA corresponding to an open reading frame of 1365 nucleotides and encoding a protein of 51,040 Da. Comparison of the mouse peptide sequences with the inferred human protein sequence revealed 88 of 99 residues to be identical. The human coding sequence of the putative enzyme was subcloned into the bacterial expression vector pMAL-c2, expressed, purified, and characterized as having guanine deaminase activity with a Km for guanine of 9.5 +/- 1.7 microM. The protein shares a 9-residue motif with other aminohydrolases and amidohydrolases (PGX[VI]DXH[TVI]H) that has been shown to be ligated with heavy metal ions, commonly zinc. The purified recombinant guanine deaminase was found to contain approximately 1 atom of zinc per 51-kDa monomer.
Highlights
Mouse erythrocyte guanine deaminase has been purified to homogeneity
The presumed coding region of this clone was used to screen a human kidney cDNA library and secondarily to polymerase chain reaction-amplify the full-length coding sequence of the human brain cDNA corresponding to an open reading frame of 1365 nucleotides and encoding a protein of 51,040 Da
Mouse erythrocytic guanine deaminase was purified to homogeneity (Fig. 1), and the amino acid sequence was obtained from five cyanogen bromide cleavage products (Table I)
Summary
Mouse erythrocyte guanine deaminase has been purified to homogeneity. The native enzyme was dimeric, being comprised of two identical subunits of approximately 50,000 Da. The human coding sequence of the putative enzyme was subcloned into the bacterial expression vector pMAL-c2, expressed, purified, and characterized as having guanine deaminase activity with a Km for guanine of 9.5 ؎ 1.7 M. Cyanogen bromide cleavage and peptide sequencing facilitated the identification and retrieval of human EST1 clones that were used to isolate a full-length guanine deaminase cDNA from human brain. The product xanthine is a substrate for xanthine oxidase in the production of uric acid It is an enzyme of purine catabolism, guanine deaminase is not ubiquitously expressed and exhibits a general absence in lymphoid tissues and variable expression elsewhere [1, 2]. The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AF095286
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