Cromoglycate markedly activates ascorbate-ADP initiated lipid peroxidation in washed rat liver microsomes.

W e have been studying a novel, reductant-dependent lipid antioxidant system in r a t liver preparations. In a preceding communication we show tha t this is triggered by nucleoside triphosphates, possibly via a G-protein mechanism. I t is selectively antagonised by cromoglycate, a nucleoside diphosphokinase inhibitor. Lipid peroxidation (LPO) was initiated non-enzymatically with 0.2 mM ascorbate in the presence of 1 mM nucleoside triphosphates or ADP and was assayed by measuring the formation of thiobarbituric reactive substances (TBARS) as in I 1 1. As a standard control, to measure the uninhibited LPO-rate, microsomes were incubated with ascorbate/ADP. W e were suprised to observe tha t inclusion of 10 mM sodium cromoglycate in these incubations enhanced this control r a t e more than 3-fold from 18 f 7 ( 12 ) to 60 f 7 ( 4 ) nmol TBARS/mg protein/30 min. There a re a number of ways in which cromoglycate might activate the ascorbate/ADP control and not all of them have ye t been explored. An interesting possibility, however, is the following. Suppose tha t microsomal adenylate kinase I 2 1 were t o convert significant amounts of ADP to AMP + ATP. In the absence of cromoglycate this ATP might be expected t o trigger the lipid antioxidant system under investigation. However, with cromoglycate present this would not be possible ( Eqn. 1 ). If this hypothetical mechanism applies, then i? preparations containing the nucleotide stimulated lipid antioxidant system, this classic LPO control would not yield a true "basal rate" for peroxidation.