Most Amino Acids Taken up by the Liver Are Converted to Carbohydrates

Most amino acids entering the liver undergo deamination, which means the amino groups are removed and the mole­cules converted to their kcto-analogues. The keto-analogues enter the pathways of carbohydrate metabolism, from which they may be completely metabolized for energy, converted to glucose or glycogen, or shunted to fatty-acid synthesis.

Deamination of amino acids for the production of carbohy­drate or energy may seem like a waste of expensive dietary protein; in some species, however, the deamination of amino acids is important for homeostasis of glucose and other fuels. For example, the natural diets of the true carnivores (e.g., cats, mink) contain a large portion of protein and little carbohydrate, but their glucose needs are no less than those of other animals, so it is extremely important that they synthesize glucose from amino acids. Ruminants are in a similar situation because most of the carbohydrates they consume undergo fermenta­tive digestion and are absorbed as volatile fatty acids rather than glucose. As with carnivores, ruminants depend on amino acids for some of their glucose needs, although a large portion of ruminant glucose requirements may be met through conversion of propionate.

To allow carbohydrate production and the deamination of excess amino acids, the endocrine reactions to high-protein meals are somewhat different from those to meals containing substantial amounts of carbohydrate. During the digestion of high-protein meals, insulin and glucagon secretion does not occur in its usual reciprocal pattern. Insulin secretion is stim-

FIGURE 32-8 Sites of entry of various amino acids into the scheme of carbohydrate metabolism. This figure illustrates the means by which glucose can be synthesized from amino acids in the process of gluconeogenesis.

ulated by amino acids as well as by glucose. Glucagon secre­tion, which is inhibited by glucose, is stimulated by amino acids as long as glucose concentrations are moderately low. This relationship means that during the digestion of a high- protein, low-carbohydrate meal, there is simultaneous secre­tion of insulin and glucagon. One of the effects of insulin is the greater cellular uptake of amino acids as well as glucose. Thus the effect of insulin in this situation is to increase transport of amino acids into tissues.

If insulin secretion were the only action stimulated by amino acid absorption, however, the animal would risk insulin- stimulated hypoglycemia when it consumed a high-protein, low-carbohydrate diet. An important action of glucagon is to

FIGURE 32-9 ■ Influence of dietary carbohydrate and protein on insulin and glucagon secretion.

stimulate gluconeogenesis through the deamination of amino acids in the liver. This process ensures that adequate glucose will be available to counterbalance the effects of amino acid- stimulated insulin secretion. Figure 32-9 illustrates the relation­ship between insulin secretion and glucagon secretion during the absorption of diets with different carbohydrate and protein concentrations.