Abstract
Abstract
This study was performed to explore basic mechanisms underlying the striking difference between the 5- to 10-fold increase in activity of mitochondrial α-glycerophosphate dehydrogenase produced by thyroid hormones in normal rat liver, and the complete lack of increase in enzyme activity in Novikoff hepatoma under similar conditions. The activity of flavin adenine dinucleotide (FAD) pyrophosphorylase, the enzyme which synthesizes the cofactor (FAD) of mitochondrial α-glycerophosphate dehydrogenase, was determined in liver and in Novikoff hepatoma grown in the rat peritoneal cavity after treatment with thyroid hormones. Thyroxine (T4) in large doses increased FAD pyrophosphorylase activity 40% in normal livers and in livers of hepatoma-bearing animals, but not in Novikoff hepatomas. Similarly, triiodothyronine increased FAD pyrophosphorylase activity in normal livers and in livers of hepatoma-bearing animals, but not in Novikoff hepatomas. These findings suggest that one mechanism which relates to the unresponsiveness of enzymes of Novikoff hepatoma to regulation by thyroid hormones may be lack of hormonal increase of FAD biosynthesis, which is required to stabilize newly formed apoenzymes.
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