4.1. Indispensable amino acids as toxic metabolites

The branched-chain amino acids (BCAAs), aromatic amino acids and sulphur amino acids all have tightly regulated oxidative pathways, which, in general have low K_M for the rate-limiting step, and consequently exist in small tissue pools (KREBS, 1972; WATERLOW and FERN, 1981). For example, the K_M for tryptophan pyrrolase is 0.15 mM, and for the branched-chain s-oxoacid dehydrogenase the KM is also at sub mM levels. As reviewed previously (MILLWARD and RIVERS, 1988), the regulatory mechanisms of these catabolic pathways include activation by feeding and an induction of their activity by the protein content of the diet. As a result of this, increases in the concentrations of the BCAAs, aromatic and sulphur amino acids on feeding are minimised. Furthermore, we know that this is a feature of these specific amino acids, and not amino acids in general. Some dispensable amino acids, such as glutamine, can be tolerated at very much larger concentrations (>20 mM; MILLWARD et al., 1982). From the perspective of these aspects of metabolic regulation, we would conclude that the organism treats many IAAs as toxic metabolites. We know that they are toxic when their concentrations rise as the result of an inability to remove them because the enzymes are lacking as in maple syrup urine disease and phenylketonuria.

Figure 4 (a). Amino acids in human skeletal muscle.

4a: Changes in IAA in human muscle at 3 hours.

Figure 4 (b). Amino acids in human skeletal muscle.

4b: Changes in IAA in human muscle at 7 hours, after a meal of 50 g of albumin. Values recalculated from BERGSTROM et al. (1990) to show the changes per kg body weight assuming that muscle water accounts for 500 mL per kg body weight.

This is not the case for ail IAAs. Lysine and threonine are different with higher KM (18 and 52 mM, respectively), larger pool sizes, (about 1 mM) and there is less evidence of the fine control of their catabolic pathways.

The difference in the handling of lysine and leucine may well be important. From this metabolic perspective we would predict that, after feeding, the branched-chain, aromatic and sulphur amino acids not incorporated into protein would be removed very rapidly, more so than lysine or threonine. Figure 4 shows data on the changes in human muscle (obtained by muscle biopsies) at 3(4a) and 7(4b) hours after a meal of 50 g of albumin (BERGSTROM, FURST and VINNARS, 1990). The increases have been calculated per kg body weight and expressed in relation to the amino acid content of the meal. For leucine and l although the intakes are the same, and although removal of these two amino acids into protein will be at the same rate (since their concentrations in protein are similar), the increase in the concentration of lysine is twice that of leucine. The same is true for threonine in comparison with valine. By seven hours, the concentration of all these IAAs had fallen below the baseline with the exception of threonine and lysine, where there was still an excess of amino acid over the baseline value.

Clearly these changes in the free pool size are a consequence of several processes which deliver them to and remove them from the tissue, and changes in oxidation with feeding are only part of those processes, but these data do confirm that the organism is not prepared to let any of the pool sizes of these IAAs expand for very long, with the exception of lysine and threonine. The implications of these differences between lysine and threonine on the one hand, and the other IAAs on the other, will be considered further below.

The concept of IAAs as toxic metabolites involves the need not only to be able to activate oxidation by feeding, but also to induce the capacity for oxidation. We know from animal experiments that the branched-chain s-oxodehydrogenase is induced by dietary protein (HARRIS et al., 1986; PATSTON et al., 1986), SO we would expect to see the human organism act in the same way.

In fact we have investigated this with studies looking at the extent to which the amount of leucine oxidation depends on the previous protein intake level. We fed diets of increasing protein from 0.35 to 2 g per kg for two weeks, and measured leucine oxidation and deposition as protein. The results are shown in Figure 5.

The increasing protein intake induced increasing leucine oxidation, as well as increasing deposition as protein. However, when we switched from the high-protein diet to a moderate intake, and measured the response after two days, the high rate of leucine oxidation was still occurring, so that all of the dietary and some of the body protein was oxidised. The previous intake had conditioned the body to expect a high-protein diet and had induced an oxidative capacity which was now too much for the lower intake. By seven days of the lower intake, the oxidative capacity had readjusted back to a lower, more appropriate level.

This demonstrates the first principle of our model, that the amino acid oxidative capacity of the organism is determined by the habitual protein intake.

Figure 5. Leucine oxidation and deposition into protein in relation to the dietary protein intake level.

Measurements involve ¹³C leucine infusions in normal adults fed the diets for 10-14 days except where indicated (MILLWARD), D.J., PACY, P.J.H., PRICE, G.M., QUEVADO, R.M., HALLIDAY, D.. Unpublished results).

Figure 6. Magnitude of diurnal cycling of body protein in relation to habitual protein intake.

Values calculated from 12-hour nitrogen balances corrected for changes in the body urea pool measured over a 48-hour period in adults habituated to the intakes for at least 14 days (MILLWARD, D.J., PACY, P.J.H., PRICE, G.M., QUEVADO, R.M., HALLIDAY, D.. Unpublished results).

Figure 7. Schematic representation of the diurnal changes in body protein stores in relation to a set-point regulation of the upper limit of body protein.

The nature of the regulation of the set-point is not understood, but is a function of height.