Problem Amino acid output by muscle in the fasting state

In the fasting state, the liver releases glucose into the circulation, as a metabolic fuel for the central nervous system and red blood cells. Some of this glucose arises from the breakdown of liver glycogen, but much arises by gluconeogenesis from amino acids. These amino acids are mainly derived from the breakdown of muscle protein. Note that, although muscle contains a large amount of glycogen, this cannot be released as glucose because muscle lacks glucose 6-phosphatase.

The lower part of Figure 9.13 shows the results of experiments in which the artery supplying the leg muscle and the femoral vein were both cannulated in a group of fasting healthy volunteers. Plasma free amino acids were measured, and the results

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Ala Gly Lys Thr His Phe Tyr Met

Figure 9.13 Arteriovenous differences in amino acids across the liver (arterio-hepatic venous difference) and skeletal muscle (arterio-femoral venous difference). A positive arteriovenous difference means uptake of amino acids by the tissue; a negative difference means output of amino acids by the tissue. From data reported by Felig P and Wahren J (1974) Federation Proceedings 33: 1092-1097.

Table 9.11 Approximate amino acid content of muscle protein mg amino acid N per g total N

Table 9.11 Approximate amino acid content of muscle protein mg amino acid N per g total N

Alanine

370

Glycine

330

Lysine

590

Threonine

280

Histidine

170

Phenylalanine

250

Tyrosine

230

Methionine

170

are shown as the arteriovenous difference. A negative value represents a net output of the amino acid from muscle.

The upper part of Figure 9.13 shows the arteriohepatic venous difference in plasma amino acids in the same subjects, and hence the uptake or release of amino acids by the liver. Although the results are not shown, there was also a net output of glucose by the liver.

Table 9.11 shows the approximate amino acid composition of muscle protein. Can you account for the difference between the amino acid output from muscle and the amino acid composition of muscle proteins?

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