Influence of hypo- and hyperthyroidism on rat liver glycerophospholipid metabolism.

The effects of hyper- and hypothyroidism on enzyme activities involved in phospholipid metabolism in the rat liver were studied. Hyperthyroidism significantly decreases activities of both microsomal acyl-CoA:glycero-3-phosphate acyltransferase (GPAT) (34%, p less than 0.01) and microsomal acyl-CoA:1-acylglycero-3-phosphocholine acyltransferase (GPCAT) (28-33%, p less than 0.01). This may contribute to the decreased proportions of certain unsaturated fatty acids found in microsomal phosphoglycerides in hyperthyroidism. Mitochondrial GPAT, phospholipase A2 and cytosol lysophospholipase are unaffected by hyperthyroidism. In contrast, hypothyroidism stimulates mitochondrial GPAT (38%, p less than 0.01) and microsomal GPCAT (14-19%) activities but decreases both mitochondrial phospholipase A2 (36%, p less than 0.01) and cytosol lysophospholipase (56%, p less than 0.01) activities. The increased GPCAT activity may contribute to the increased proportions of certain unsaturated fatty acids found in microsomal phosphoglycerides in hypothyroidism. Triiodothyronine (T3) treatment of the hypothyroid rat (25 micrograms/100 g body weight/day for four days) corrected phospholipase A2 and lysophospholipase activities to the level of the control rat, but failed to correct the increased mitochondrial GPAT activity and not only corrected but lowered GPCAT activity to the level of the hyperthyroid rat.