Influences of some low molecular compounds on enzymatic activity and isoelectric point of aspartate aminotransferase from rat liver.

The substances responsible for regulating cytosolic aspartate aminotransferase (AspATc) activity in the cytosolic fraction of rat liver were examined. AspATc was removed from the cytosolic fraction by passing the fraction through an affinity column to which anti-AspATc antiserum was conjugated. The unbound fraction from the column was found to decrease the activity of the purified AspATc. A fraction containing compounds of less than MW 1,000 was obtained by filtering the cytosolic fraction through a YM 2 membrane. This YM 2 filtrate decreased the activity of the purified enzyme; however, the enzymic activity was protected partially by the addition of 2-oxoglutarate or pyridoxal phosphate (PLP). The YM 2 filtrate also decreased the isoelectric points (pIs) of the purified enzyme. Influences of glucose and fructose on AspATc were examined, and fructose was found to decrease the enzymic activity and the pIs. Fructose was more effective on apoenzyme than holoenzyme, suggesting that fructose may bind to the Lys258 residue of AspATc which is the binding site of PLP. The effects of various amino acids including substrates on the enzymic activity were also examined. Some amino acids were found to decrease the enzymic activity to various extents, though the pIs were unaltered. These results suggest that under physiological conditions, AspATc activity is modified by various low molecular substances in various ways.