Lysinearginine antagonism

The lysine-arginine antagonism provides a distinctive, but not unique, example of how one amino acid may reduce the efficiency of utilization of another. The results of one such study (D'Mello and Lewis, 1970) are shown in Table 14.2. A basal diet marginally deficient in arginine but adequate in lysine was used. Addition of excess lysine to this basal diet to give concentrations of 13.5, 16.0 and 18.5 g kg-1, progressively reduced growth performance and enhanced the quantity of arginine required to reverse these adverse effects. Thus, at lysine concentrations of 16.0 and 18.5 g kg-1 diet arginine requirements had increased from 10.0 to 14.5 and 16.0 g kg-1 diet, respectively. These increases in arginine requirements remained prominent even after responses were considered in relation to arginine intake (Fig. 14.16). The discrete response curves and the changes in slope indicate that arginine utilization is markedly reduced by excess dietary lysine.

The response of growing chicks to the lysine-arginine interaction may be modified by at least two dietary factors. Supplementation with electrolytes, particularly potassium acetate, reduces the severity of this antagonism (O'Dell

Table 14.2. Effects of dietary lysine and arginine on daily weight gain (g) of chicks. (From D'Mello and Lewis, 1970.)

Dietary lysine (g kg 1) Dietary arginine -

Table 14.2. Effects of dietary lysine and arginine on daily weight gain (g) of chicks. (From D'Mello and Lewis, 1970.)

8.5

17.7

16.1

13.0

7.2

10.0

19.6

19.5

17.1

14.0

11.5

19.8

20.3

18.4

15.6

13.0

19.8

19.3

19.1

17.2

14.5

19.9

19.4

20.3

19.0

16.0

19.5

19.4

19.4

19.8

0 100 200 300 400 500 600

Arginine intake (mg day-1)

Fig. 14.16. Daily growth rates and arginine intake of chicks fed 11.0 (•), 13.5 (o), 16.0 (A) and 18.5 (A) g lysine kg 1 diet. (From D'Mello, 1973c. Reproduced with permission of British Poultry Science Ltd.)

and Savage, 1966). A second factor influenc- in the diet. Canavanine is one such analogue ing the lysine-arginine antagonism is the pres- occurring naturally in the seed of the legume ence of other structural analogues of arginine Canavalia ensiformis. The existence of a canavanine-arginine antagonism in chicks fed this seed has been proposed, with lysine erbating the deleterious effects of this interaction (D'Mello et al, 1989; see also Chapter 7).

The practical significance of the lysine-arginine antagonism emanates not only from its relevance to the assessment of arginine requirements of growing poultry but also from its effects in determining the nutritive value of certain feedingstuffs. Thus, Miller and Kifer (1970) noted that the nutritive value of an aged sample of fishmeal could be enhanced by arginine supplementation and impaired by addition of lysine or methionine. Leslie et al. (1976) reported the existence of an adverse lysine:arginine ratio in rapeseed meal when it served as the sole source of dietary protein. Under these conditions, arginine supplementation improved the nutritive value of rape-seed meal whereas lysine addition precipitated severe arginine-responsive growth depressions. Arginine supplementation is also beneficial when chicks are fed diets containing single-cell protein sources such as hydrocarbon-grown yeast (D'Mello, 1973b) and methanol-grown bacteria (D'Mello, 1978). Another interesting feature of the bacterial source has been the observation that chicks also respond to supplementation with electrolytes, which act by reducing mortality (Talbot, 1978). It remains to be established whether this represents another dimension of the lysine-arginine-electrolyte interactions discussed earlier.

Branched-chain amino acid antagonisms

Several studies with the young chick and the turkey poult illustrate clear patterns of interdependence in the metabolism of and requirements for BCAA. For example, dietary leucine exerts a profound effect on the valine requirements of the chick. Thus, for leucine concentrations of 14, 24 and 34 g kg-1 diet, valine requirements are 7.7, 8.9 and 10.1 g kg-1 diet, respectively (D'Mello and Lewis, 1970). The leucine-isoleucine antagonism has also been described in quantitative terms. Increasing leucine levels from 14 to 21.5 and 29.0 g kg 1 diet enhances isoleucine requirements of the chick from 5.8 to 6.2 and 6.5 g kg-1

diet, respectively (D'Mello and Lewis, 1970). It should be noted that the leucine-isoleucine antagonism is considerably less potent than that between leucine and valine, a feature alluded to by D'Mello and Lewis (1970). Consequently, the observation by Burnham et al. (1992) that dietary leucine at 1.76 times requirement depressed chick growth without enhancing isoleucine requirements is consistent with the weak antagonism between these two amino acids. The complexity of antagonistic effects among the BCAA is further illustrated by the impact of dietary isoleucine on valine and leucine requirements. Thus, an isoleucine concentration of 5.2 g kg-1 diet permitted satisfactory chick growth and efficiency of food utilization with the concentrations of leucine and valine set at 9.8 and 6.3 g kg-1 diet, respectively. However, an isoleucine concentration of 7.6 g kg-1 diet increased requirements for leucine and valine to 11.0 and 7.5 g kg-1 diet respectively (D'Mello, 1974). The extent to which BCAA utilization is affected by mutual antagonisms may be gauged by the growth responses of turkey poults to excess leucine (Fig. 14.17). As the leucine content is increased from 14.2 to 20.2 g kg 1 diet, there is a positive displacement in the response curve to valine, indicating that valine utilization is impaired by excess leucine (D'Mello, 1988). The lower plateau in the response curve to excess leucine suggests that isoleucine may now be the limiting factor.

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