In mammalian tissues cardiolipin is rapidly remodeled by monolysocardiolipin acyltransferase subsequent to its de novo biosynthesis (Ma, B. J., Taylor, W. A, Dolinsky, V. W., and Hatch, G. M. (1999) J. Lipid Res. 40, 1837-1845). We report here the purification and characterization of a monolysocardiolipin acyltransferase activity from pig liver mitochondria. Monolysocardiolipin acyltransferase activity was purified over 1000-fold by butanol extraction, hydroxyapatite chromatography, and preparative SDS-PAGE. The purified 74-kDa protein catalyzed acylation of monolysocardiolipin to cardiolipin with [(14)C]linoleoyl coenzyme A. Photoaffinity labeling of the protein with 12-[(4-[(125)I]azidosalicyl)amino]dodecanoyl coenzyme A indicated coenzyme A was bound at its active site and photoaffinity cross-linking of 12-[(4-azidosalicyl)amino]dodecanoyl coenzyme A to the enzyme inhibited enzyme activity. Enzyme activity was optimum at pH 7.0, and the enzyme did not utilize other lysophospholipids as substrate. The purified enzyme was heat-labile and exhibited an isoelectric point of pH 5.4. To determine the enzymes kinetic mechanism the effect of varying concentrations of linoleoyl coenzyme A and monolysocardiolipin on initial velocity were determined. Double-reciprocal plots revealed parallel lines consistent with a ping pong kinetic mechanism. When the enzyme was incubated in the absence of monolysocardiolipin, coenzyme A was produced from linoleoyl coenzyme A at a rate consistent with the formation of an enzyme-linoleate intermediate. The true K(m) value for linoleoyl coenzyme A and true K(m) value for monolysocardiolipin were 100 and 44 microM, respectively. The calculated V(max) was 6802 pmol/min per mg of protein. A polyclonal antibody, raised in rabbits to the purified protein, cross-reacted with the protein in crude pig liver mitochondrial fractions. In liver mitochondria prepared from thyroxine-treated rats, the level of the protein was elevated compared with euthyroid controls indicating that expression of monolysocardiolipin acyltransferase is regulated by thyroid hormone. The study represents the first purification and characterization of a monolysocardiolipin acyltransferase activity from any organism.