Abstract
Extract: Triodothyronine (T3) was given daily by injection to rats from 1 day of age to 2 hr prior to death. In the brain microsomal fraction of 5− to 6-day-old animals, incorporation of malonyl-CoA into total fatty acids increased from 0.9 to 2.25 mμmoles/mg protein. Although thyroid increased precursor incorporation into total fatty acids in other age groups, the difference was not statistically significant. Increased precursor incorporation was entirely due to increased formation of saturated fatty acids, and this effect was observed at all age groups.
Incorporation of acetyl-CoA into saturated and unsaturated fatty acids by the mito-chondrial fraction was not increased significantly by administration of T3.
In rats made hypothyroid at birth, fatty acid chain elongation was reduced in both the microsomal and the mitochondrial fractions. At 6 days of age, incorporation of malonyl-CoA by the microsomal fraction fell from 1.41 to 0.17 mμmoles/mg protein. These studies suggest that thyroid stimulates the microsomal-fatty acid-brain elongation system involved in the synthesis of saturated fatty acids characteristic of myelin lipids.
Speculation: The stimulation of fatty acid biosynthesis by thyroid hormone is specifically limited to the synthesis of saturated fatty acids by the microsomal fraction. Since the effect of thyroid is probably mediated by enzyme synthesis, we conclude that the microsomal fraction contains two distinct enzyme systems involved in fatty acid chain elongation, one for the formation of polyunsaturated fatty acids, the other for the formation of saturated fatty acids. The activity of the latter system increases at time of myelination, and as a consequence of thyroid stimulation.
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Grippo, J., Menkes, J. Effect of Thyroid on Fatty Acid Biosynthesis in Brain. Pediatr Res 5, 466–471 (1971). https://doi.org/10.1203/00006450-197109000-00004
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DOI: https://doi.org/10.1203/00006450-197109000-00004