Partial purification and kinetic properties of human placental cytosolic aspartate transaminase

Human placental cytoplasmic aspartate transaminase was purified 404-fold by heat treatment, ammonium sulfate fractionation, dialysis and DEAE-Sephadex chromatography. The pH optimum of the enzyme was 6.8 in either phosphate or cacodylate buffer. The Km values of alpha-ketoglutarate and L-aspartate were 2.06 and 22.5 mM, respectively. A 78% inhibition of the enzyme was noted at 4 mM concentration of maleate which inhibited the enzyme upon competing with alpha-ketoglutarate with a Ki value of 1.72 mM. The kinetic properties of this enzyme are compared with those of the enzyme from various mammalian and other sources. The data are discussed in terms of the probable effectiveness of this enzyme in catabolizing L-aspartate in placenta especially after the consumption of a high protein diet by the pregnant mother.