The Metabolism of 2,5‐Diphenyloxazole (PPO) in Human Lymphocytes and Rat Liver Microsomes

Abstract

The oxidative metabolism of 2,5-diphenyloxazole (PPO) is associated with 3-methylcholanthrene inducible cytochrome P-450. The major metabolite formed has m/z of 237, corresponding to hydroxylated PPO. All the possible hydroxylated metabolites of PPO were synthesized and characterized, enabling the assignment of a structure for the major metabolite and two minor metabolites. The metabolites are easily extracted and their fluorescence is quantifiable in alkaline medium with a sample fluorescence to blank fluorescence ratio of 400:1. A sensitive HPLC assay of PPO metabolism was also developed. PPO metabolism is readily catalyzed by 3-methylcholanthrene-induced rat liver microsomes and strongly inhibited by alpha-naphthoflavone, but poorly inhibited by metyrapone or SKF 525A, indicating the involvement of cytochrome P-448 or P1-450 in the metabolism of PPO. With human lymphocytes the method has proven to be a good indicator of "aryl hydrocarbon hydroxylase" (AHH) activity, correlating well with AHH assays using benzo(alpha)pyrene (BP) as a substrate. Both the induced BP and PPO metabolism by human lymphocytes is inhibited by alpha-naphthoflavone, but not by metyrapone.

Cytochrome P-450 may link intracellular Ca2+ stores with plasma membrane Ca2+ influx

“…We found extremely slow arachidonic acid metabolism under both sets of conditions, which is consistent with previous observations in this cell type [21]. Within the cytochrome P-450 family, inhibition bya-naphthoflavone, but not by metyrapone or 4-hydroxyandrostenedione, suggests the involvement of cytochrome P1-450 or P-448 [22][23][24]. These cytochromes have been found in microsomes of human lymphocytes, where they are responsible for enzymic activities such as aryl hydrocarbon hydroxylase [22,[25][26][27].…”

“…Within the cytochrome P-450 family, inhibition bya-naphthoflavone, but not by metyrapone or 4-hydroxyandrostenedione, suggests the involvement of cytochrome P1-450 or P-448 [22][23][24]. These cytochromes have been found in microsomes of human lymphocytes, where they are responsible for enzymic activities such as aryl hydrocarbon hydroxylase [22,[25][26][27]. Our results therefore suggest that microsomal cytochrome P-450 is involved in the increase in the plasma membrane permeability to Ca2+ observed when the Ca2+ stores are empty.…”

Two novel 2,5-diphenyl oxazole derivatives from Gymnotheca chinensis

“…This kind of alkaloid was found for the first time from this endemic genus. 2,5-Diphenyl oxazole (DPO) was a class of substances exhibiting radioprotective and radiosensitizing effects [8,9]. This type of compound is very rare in the natural world.…”