Purification and properties of mitochondrial uracil-DNA glycosylase from rat liver

Abstract

Uracil-DNA glycosylase from rat liver mitochondria, an inner membrane protein, has been purified approximately 575000-fold to apparent homogeneity. During purification two distinct activity peaks, designated form I and from II, were resolved by phosphocellulose chromatography. Form I constituted .apprx.85% while form II was .apprx.15% of the total activity; no interconversion between the forms was observed. The major form was purified as a basic protein with an isoelectric point of 10.3. This enzyme consists of a single polypeptide with an apparent Mr of 24000 as determined by recovering glycosylase activity from a sodium dodecyl sulfate-polyacrylamide gel. A native Mr of 29000 was determined by glycerol gradient sedimentation. The purified enzyme had no detectable exonuclease, apurinic/apyrimidinic endonuclease, DNA polymerase, or hydroxymethyluracil-DNA glycosylase activity. A 2-fold preference for single-stranded uracil-DNA over a duplex substrate was observed. The apparent Km for uracil residues in DNA was 1.1 .mu.M, and the turnover number is about 1000 uracil residues released per minute. Both free uracil and apyrimidinic sites inhibited glycosylase activity with Ki values of approximately 600 .mu.M and 1.2 .mu.M, respectively. Other uracil analogues including 5-(hydroxymethyl)uracil, 5-fluorouracil, 5-aminouracil, 6-azauracil, and 2-thiouracil or analogues of apyrimidinic sites such as deoxyribose and dexoyribose 5''-phosphate did not inhibit activity. Both form I and form II had virtually identical kinetic properties, and the catalytic fingerprints (specificity for uracil residues located in a defined nucleotide sequence) obtained on a 152-nucleotide restriction fragment of M13mp2 uracil-DNA were almost identical. These properties differentiated the mitochondrial enzyme from that of the uracil-DNA glycosylase purified from nuclei of the same source.