3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity and chemical composition of rat palatal tissue after gamma irradiation.

The effect of various amounts of head and neck irradiation on the enzyme activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (E.C. 1.1.1.34) from rat palatal tissue was measured. Enzyme activity increased with increasing radiation exposure, showing a plateau at 800–1200 rads with about five times the original activity. Above 1200 rads, the enzyme activity returned to its previous range. It is suggested that irradiation may interfere directly or indirectly with some enzyme regulatory mechanisms.The DNA, RNA, total protein and total lipid content of the irradiated (1200 rads) and sham-irradiated rat palatal tissues were determined. Of these parameters, the total lipid was the only one significantly altered, with about a two-fold increase and with its cholesterol content greatly increased.As the liver is known to have received scattered radiation, (～-5 per cent) the enzyme activity and total cholesterol were also determined in the liver of rats that received irradiation to the neck. These values when compared to those obtained from the livers of the sham-irradiated rats showed a 2.5-fold increase in the enzyme levels of 3-hydroxy-3-methylglutaryl-coenzyme A reductase while its total cholesterol doubled.