Time-dependence of inhibition of carnitine palmitoyltransferase I by malonyl-CoA in mitochondria isolated from livers of fed or starved rats. Evidence for transition of the enzyme between states of low and high affinity for malonyl-CoA

Publisher Website
Google Scholar
Abstract
The degree of inhibition of CPT I (carnitine palmitoyltransferase, EC 2.3.1.21) in isolated rat liver mitochondria by malonyl-CoA was studied by measuring the activity of the enyzme over a short period (15 s) after exposure of the mitochondrial to malonyl-CoA for different lengths of time. Inhibition of CPT I by malonyl-CoA was markedly time-dependent, and the increase occurred at the same rate in the presence or absence of palmitoyl-CoA (80 .mu.M), and the presence of carnitine, such that the time-course of acylcarnitine formation deviated markedly from linearity when CPT I activity was measured in the presence of malonyl-CoA over several minutes. The initial rate of increase in degree of inhibition with time was independent of malonyl-CoA concentration. CPT I in mitochondria from 48 h-starved rats had a lower degree of inhibition by malonyl-CoA at zero time, but was equally capable of being sensitized to malonyl-CoA, as judged by an initial rate of increase of inhibition identical with that of the enzyme in mitochondrial from fed rats. Double-reciprocal plots for the degree of inhibition produced by different malonyl-CoA concentrations at zero time for the enzyme in mitochondria from fed or starved animals indicated that the enzyme in the latter mitochondria was predominantly in a state with low affinity for malonyl-CoA (concentration required to give 50% inhibition, I0.5 .simeq. 10 .mu.M), whereas that in mitochondria from fed rats displayed 2 distinct sets of affinities:low (.simeq. 10 .mu.M) and high (< 0.3 .mu.M). Plots for mitochondria after incubation for 0.5 or 1 min with malonyl-CoA indicated that the increased sensitivity observed with time was due to a gradual increase in the high-affinity state in both types of mitochondria. The sensitivity of CPT I in rat liver mitochondria in vitro had 2 components: an instantaneous sensitivity inherent to the enzyme which depends on the nutritional state of the animal from which the mitochondria are isolated and a slow, malonyl-CoA-induced, time-dependent increase in sensitivity. The rate of malonyl-CoA-induced sensitization of the enzyme to malonyl-CoA inhibition is probably limited by a slow 1st-order process, which occurs after the primary event of interaction of malonyl-CoA with the mitochondria. The putative role of such a process in a possible mechanism whereby differences in sensitivity in vivo (related to differences in hepatic malonyl-CoA concentrations) may be partially retained in vitro is discussed.

This publication has 12 references indexed in Scilit: