Hepatic mitochondrial inner membrane properties and carnitine palmitoyltransferase A and B. Effect of diabetes and starvation

Abstract

Intact mitochondria and inverted submitochondrial vesicles were prepared from the liver of fed, starved (48 h) and streptozotocin-diabetic rats in order to characterize carnitine palmitoyltransferase kinetics and malonyl-CoA sensitivity in situ. In intact mitochondria, both starved and diabetic rats exhibited increased Vmax., increased Km for palmitoyl-CoA, and decreased sensitivity to malonyl-CoA inhibition. Inverted submitochondrial vesicles also showed increased Vmax. with starvation and diabetes, with no change in Km for either palmitoyl-CoA or carnitine. Inverted vesicles were uniformly less sensitive to malonyl-CoA regardless of treatment, and diabetes resulted in a further decrease in sensitivity. In part, differences in the response of carnitine palmitoyltransferase to starvation and diabetes may reside in differences in the membrane environment, as observed with Arrhenius plots, and the relation of enzyme activity and membrane fluidity. In all cases, whether rats were fed, starved or diabetic, and whether intact or inverted vesicles were examined, increasing membrane fluidity was associated with increasing activity. Malonyl-CoA was found to produce a decrease in intact mitochondrial membrane fluidity in the fed state, particularly at pH 7.0 or less. No effect was observed in intact mitochondria from starved or diabetic rats, or in inverted vesicles from any of the treatment groups. Through its effect on membrane fluidity, malonyl-CoA could regulate carnitine palmitoyltransferase activity on both surfaces of the inner membrane through an interaction with only the outer surface.

Influence of diet on the kinetic behavior of hepatic carnitine palmitoyltransferase I toward different acyl CoA esters

“…Higher values of K 0.5 for palmitoyl CoA have been reported in mitochondria from the liver of rats subjected to starvation (17,18), indicating that dietary manipulation can lead to higher K 0.5 values. It should be noted, however, that K 0.5 values from different laboratories are not strictly comparable due to differences in the albumin concentrations used in the assay, the level of which would modify the free concentration of acyl CoA available to CPT I.…”

Carnitine: a nutritional, biosynthetic, and functional perspective

“…Although changes in malonyl-CoA sensitivity are considered the major mechanism for the regulation of the liver isoform of CPT-I and thus for hepatic mitochondrial fatty acid oxidation, the mechanism for the decrease in malonyl-CoA sensitivity has not been uncovered. It has been suggested that changes in the lipid composition of the membrane microenvironment in which CPT-I resides are important for alteration in malonyl-CoA sensitivity (Brady et al, 1985;Zammit et al, 1998;Mynatt et al, 1994). Alternatively, covalent modification of CPT-I protein itself could also be responsible for these changes in the enzymes' malonyl-CoA sensitivity.…”

Co-ordinate induction of hepatic mitochondrial and peroxisomal carnitine acyltransferase synthesis by diet and drugs

“…The CPT on the outside of mitochondria has been described as the rate-limiting enzyme in hepatic mitochondrial /-oxidation of fatty acids (McGarry & Foster, 1980). and our previous data show that hepatic CPT activity (outer, inner and total) increases in starvation, diabetes and riboflavin deficiency, and that the increased activity is correlated with increased CPT mRNA translation, increased CPT mRNA levels, and increased transcription of the CPT mRNA from DNA for the 68 kDa mitochondrial CPT (Brady et al, 1985(Brady et al, , 1988Brady & Brady, 1987, 1989a. CPT activity (total) also increases with increased dietary fat (Brady et al, 1986b;Berge et al, 1987), and CPT activity and synthesis increase with DEHP, acetylsalicylic acid and clofibrate administration Brady & Brady, 1989b).…”