Age Dependence of Thyroxine Stimulation of Riboflavin Incorporation into Flavin Coenzymes in Liver and Brain*

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Abstract
The incorporation of radioactive riboflavin ([14C]riboflavin) into [14C]FMN and [14C]FAD was determined in livers of thyroxine-treated, control and hypothyroid rats 1 h after an s.c. injection. In normal animals the incorporation of radioactive riboflavin into hepatic FMN and FAD was relatively low at birth, increased from birth to 16 days, and remained relatively stable for up to 7 mo. Daily thyroxine treatment (300 .mu.g/100 g BW [body weight]) of adult rats for 7 days increased incorporation into [14C]FMN and [14C]FAD by 50-80%, whereas radiothyroidectomy diminished incorporation into both coenzymes by 20%. In newborn rats, daily thyroxine (10 .mu.g/animal) increased incorporation into [14C]FMN and [14C]FAD by 25-40% at 8 days of age; radiothyroidectomy of newborn rats reduced incorporation into [14C]FAD by 30% at 15 days of age. The incorporation of [14C]riboflavin into [14C]FAD in cerebrum and cerebellum was also age-dependent, being lowest at birth and greater from 16 days to 7 mo. of age. In cerebrum and cerebellum of 8 day old rats treated with thyroxine from birth, the incorporation of [14C]riboflavin into [14C]FAD was increased by 25-50%; radioactive iodine treatment of newborn rats reduced incorporation into [14C]FAD by 50% in cerebrum and in cerebellum at 15 days of age. By contrast, in brains of adult animals, [14C]riboflavin incorporation into [14C]FAD was not increased by thyroid hormones; in brains of adult hypothyroid rats, [14C]-incorporation into [14C]FAD was increased rather than decreased. Thyroid hormones enhance incorporation of riboflavin into FAD in livers of both adult and newborn animals, but in cerebrum and cerebellum of newborn animals only. The lack of enhancement of flavin metabolism in brains of adult animals by thyroxine is compatible with other known evidence of resistance of the brain to stimulation by thyroid hormones.