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Vitamin E metabolism in poultry
Dietary vitamin E accumulates poorly in the muscle of turkeys compared to chickens. This is likely one of the reasons oxidative rancidity is more of a problem in turkey compared to chicken meat. Enhanced metabolism of vitamin E isomers to water soluble degradation products may be responsible for diminished vitamin E levels in turkey muscle. Cytochrome P450 hydroxylases have been shown to initiate this type of vitamin E metabolism in humans and rodents yet little work has been done in agricultural animals. We have investigated the hydroxylase-mediated metabolism pathway in bile from turkeys and chickens receiving equivalent levels of dietary vitamin E isomers. Metabolites of alpha tocopherol (aT) and gamma tocopherol (gT) were enhanced up to 8-fold in turkey compared to chicken bile. This suggested that enhanced metabolism of vitamin E in turkeys can partly explain the poor accumulation in muscle. We also utilized RNA Seq to characterize transcription levels of genes in liver tissue of the chickens and turkeys. Gene expression of four cytochrome P450s were up-regulated in turkey compared to chicken (3.5 to 10.3-fold). One of these P450s may be a tocopherol hydroxylase. Gene expression of a P450 associated with cholesterol synthesis was elevated 3.5-fold in turkey liver which may explain our observation of elevated cholesterol in turkey bile. Cholesterol concentration in bile has relevance in regard to vitamin E uptake in the intestine since cholesterol in the intestinal lumen has been shown to block vitamin E absorption by enterocytes. Bile containing cholesterol dumps into the intestinal lumen during feeding. Gene expression of a P450 associated with bile salt synthesis was elevated 10.3-fold in turkey liver. Excess bile salt production in the liver may facilitate transfer of hepatic cholesterol to the bile by forming a colloidal interaction with the sterol. We observed gT was up to 9.7-fold higher in turkey bile which may also be mediated by excess bile salts in liver that are exported to the bile and carry gT as cargo. Gene expression of UDP-glucuronyl transferase (UGT) was up-regulated 3.3-fold in turkey liver which presents an additional mechanism that may limit vitamin E accumulation in turkey muscle. Sequence variation in the alpha tocopherol transfer protein (ATTP) is noted between turkey and chicken at sites known to affect its transfer function. Thus, the interplay between bile salts, cholesterol, ATTP, UGT, and tocopherol hydroxylases can influence vitamin E transport and metabolism in poultry.
Keywords: tocopherol, poultry, metabolism