Human placental lipid peroxidation. Some characteristics of the NADPH-supported microsomal reaction.

1. The evidence presented in this paper indicates the existence of NADPH-supported lipid peroxidation in human placental microsomes. Thiobarbituric acid assay was used to estimate quantitatively lipid peroxidation. 2. Several biochemical characteristics of the reaction were examined. Maximal lipid peroxidation occurred at pH 7.4 and at a protein concentration of approx. 0.2 mg microsomal protein/ml. The presence of NADPH and chelated iron was required. The reaction was linear up to 5 min and did not exhibit an initial lag phase. 3. Under optimal assay conditions, the rate of lipid peroxidation ranged from 2 to 6 nmol malondialdehyde formed/min/mg protein in different preparations of placental microsomes. 4. Inconclusive results were obtained when assays were performed in the presence of scavengers of reactive oxygen species. 5. Marked inhibition in the malondialdehyde accumulation was observed when phosphate buffer was added to the incubation media. 6. This inhibitory effect appeared to be due to the removal of chelated iron from the system and not due to interference with the electron transport mechanism.