No abstract

The role of biotin-dependent enzymes in the fatty liver and kidney syndrome of young chicks was studied. Under conditions of a marginal deficiency of dietary biotin, the level of biotin in the liver has differing effects on the activities of two biotin-dependent enzymes, pyruvate carboxylase and acetyl-CoA carboxylase. The activity of acetyl-CoA carboxylase is increased, but when the dietary deficiency of biotin produces biotin levels which are below o· 8 p,g/g of liver, the activity of pyruvate carboxylase may be insufficient to completely metabolize pyruvate via gluconeogenesis. There is an increase in liver size and in the activities of enzymes involved in alternate pathways for the removal of pyruvate. Blood lactate accumulates and there is increased synthesis of fatty acids, and an accumulation of palmitoleic acid; these steps are accomplished by increased activities of at least the following enzymes: acetyl-CoA carboxylase, malate dehydrogenase (decarboxylating) (NADP+) and the desaturase enzyme. When the biotin level is below 0·35 p,g/g of liver and the chick is subjected to a stress, physiological defence mechanisms of the chick may be inadequate to maintain homeostasis and they finally collapse, resulting in accumulation of triacylglycerol in the liver and blood; the chick is unable to maintain blood glucose levels and death occurs, often only a few hours after the imposition of the stress.

Fatty liver and kidney syndrome, a disorder of young chicks, was studied under laboratory conditions. Affected chicks had enlarged livers (hepatomegaly), an increased content of lipid in the liver, and an increased level of palmitoleic acid in the liver lipids. The disorder was observed mainly in chicks from young parent flocks, and was associated either with commercial diets which were subsequently found to be low in biotin, or with specially formulated low-biotin diets. A third factor, imposition of stress, was required to initiate the disorder. There was evidence of increased lipogenesis causing an increase of triacylglycerols in the liver lipids and an increased production of saturated fatty acids, particularly palmitic acid. Increased levels of palmitoleic acid resulted from an increased desaturation of palmitic acid. Under stress, affected chicks had low blood glucose levels, suggesting that gluconeogenesis was impaired. Since biotin-dependent enzymes are involved in both gluconeogenesis and lipogenesis, it would appear that the relevant enzymes respond differently to a deficiency of biotin.