The influence of CCl4 biotransformation on the activation of rat liver phospholipase C in vitro.
Carbon tetrachloride (CCl4) biotransformation and covalent binding was measured in 1000g liver fractions by determining the amount of 14CCl4 metabolites covalently bound to proteins and lipids at various (5-60 min) incubation times. Reactive intermediate binding to proteins and phospholipids peaked at 20 min, whereas CCl4 metabolites associated with neutral lipids (primarily diacylglycerol) were initially low (0-15 min) and then gradually increased from 20 to 60 min. The rise in labeled diacylglycerol was associated with a decrease in phospholipids containing covalently bound CCl4 metabolites, since CCl4 bioactivation increased phospholipase C (PLC) activity three- to fourfold. The major rise in PLC activity occurred after the plateau of CCl4 metabolite binding to cellular phospholipids. In contrast, when CCl4 bioactivation is absent, 0.5 mM CCl4 has little effect on PLC activity. At CCl4 concentrations of 1 mM and greater, the NADPH-dependent activation of PLC by CCl4 is reduced because CCl4 biotransformation is inhibited. Nevertheless, PLC is still activated by CCl4; however, PLC activation by high doses of CCl4 occurs by bioactivation-independent mechanisms. Therefore, there are two components of CCl4-induced PLC activation: one which is dependent on CCl4 biotransformation and one which is not. Under both conditions (+/- biotransformation), the activation of PLC may be a key event in CCl4 hepatotoxicity since PLC disrupts the functional and structural integrity of membranes by degrading membrane phospholipids.
Department of Pharmacology, Medical College of Virginia, Richmond 23298-0217.
J B
JB
Coleman
L W
LW
Condie
R G
RG
Lamb
D000818:Animals; D001711:Biotransformation; D002118:Calcium / Q000378:metabolism; D002251:Carbon Tetrachloride / Q000493:pharmacokinetics*; D004789:Enzyme Activation; D008099:Liver / Q000201:enzymology*; D008297:Male; D008797:Metyrapone / Q000494:pharmacology; D009249:NADP / Q000378:metabolism; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D010738:Type C Phospholipases / Q000378:metabolism*
D011725:Pyridines; C013109:metapyrone; D009249:NADP; D002251:Carbon Tetrachloride; D010738:Type C Phospholipases; D002118:Calcium; D008797:Metyrapone