Role of carnitine palmitoyltransferase I in the regulation of hepatic ketogenesis during the onset and reversal of chronic diabetes

Abstract

1. The kinetic properties of overt carnitine palmitoyltransferase (CPT I, EC 2.3.1.21) were studied in rat liver mitochondria isolated from untreated, diabetic and insulin-treated diabetic animals. A comparison was made of the time courses required for the changes in these properties of CPT I to occur and for the development of ketosis during the induction of chronic diabetes and its reversal by insulin treatment. 2. The development of hyperketonaemia over the first 5 days of insulin withdrawal from streptozotocin-treated rats was accompanied by parallel increases in the activity of CPT I and in the I0.5 (concentration required to produce 50% inhibition) of the enzyme for malonyl-CoA. 3. The rapid reversal of the ketotic state by treatment of chronically diabetic rats with 6 units of regular insulin was not accompanied by any change in the properties of CPT I over the first 4 h. Higher doses of insulin (15 units), delivered throughout a 4 h period, resulted in an increase in the affinity of CPT I for malonyl-CoA, but the sensitivity of the enzyme to the inhibitor was still significantly lower than in mitochondria from normal animals. 4. Conversely, when insulin treatment was continued over a 24 h period, full restoration of the sensitivity of the enzyme to malonyl-CoA was achieved. However, the activity of the enzyme was only decreased marginally. 5. These results are discussed in terms of the possibility that the major regulatory sites of the rate of hepatic oxidation may vary in different phases of the induction and reversal of chronic diabetes.

Role of insulin in hepatic fatty acid partitioning: emerging concepts

“…Interestingly, the expression of CPT I protein showed a much more gradual decline than that of its mRNA, indicating that the protein has a long half-life. The same inference was drawn from measurement of CPT I activity in mitochondria isolated from livers of starved-refed [66] or insulin-treated streptozotocin-diabetic rats [67].…”

“…This is considerably later than the large decrease in plasma insulin concentrations that occurs after 6 h of starvation [68]. Similarly, reversal of the effects of starvation or streptozotocininduced diabetes in rats only starts 6 h after refeeding of the animals and is not complete even after 12 h after the start of refeeding [66] or insulin treatment [67] respectively. The requirement for several hours to achieve the observed modification of CPT I kinetics has been confirmed in experiments using cultured hepatocytes exposed to insulin or glucagon [63,69] and in rats subjected to a hyperinsulinaemic\euglycaemic clamp [70].…”

“…It also has important implications for the distribution of metabolic control over the fatty acid oxidative pathway, especially during the reversal of oxidative (ketogenic) conditions. When starved rats are refed [66] or when diabetic animals are treated with insulin [67], there is a rapid decline in circulating ketone-body concentrations, reflecting a decreased rate of ketone-body production in the liver. The questions arise, therefore, as to the involvement of insulin in this response, as well as to the site at which the effect is exerted.…”

“…If, as the enzymological data appear to indicate, the expression of CPT I remains high in spite of the rapid decline in CPT I mRNA (because of the long half-life of the protein) and its desensitized state with respect to malonyl-CoA inhibition persists during the reversal of ketogenic states [66,67], then it would be anticipated that this reaction no longer exerts major control over the rate of fatty acid oxidation during these metabolic transitions. It has been suggested that intramitochondrial 3-hydroxy-3-methylglutaryl-CoA (mHMG-CoA) synthase becomes an important locus of control under these conditions [71,72].…”

The malonyl-CoA–long-chain acyl-CoA axis in the maintenance of mammalian cell function

“…It is to be emphasized that the sensitivity of L-CPTo to malonyl-CoA, as well as the K m for palmitoyl-CoA, are both modulated by physiological state [45][46][47][48] (but not in cardiac muscle [49]) and that these changes (i) can be mimicked by experimentally induced changes in the fluidity of the mitochondrial outer membrane in itro [50] and (ii) are correlated with changes in the molecular order of the lipids of outer membranes prepared from mitochondria isolated from livers of rats maintained under different physiological conditions [51]. It is inferred that protein-membrane interactions are likely to be important in determining the malonyl-CoA sensitivity of CPTo, and that they are likely to be due to interaction between the TM segments as well as between them and the annular membrane lipids [34].…”