Bioavailability of DMethionine and DCysteine

Unlike most other amino acids, which are produced commercially by fermentation, crystalline methionine is produced by chemical synthesis. This has an important biological implication because whereas fermentation yields only the natural l-isomer, chemical synthesis yields a racemic (50:50) mixture of d-and l-isomers (dl-methionine). o-Amino acids are not used by animals for protein synthesis or for other metabolic purposes. Therefore, any ingested d-methionine must be converted into l-methionine before it can be utilized. The conversion (or inversion) consists of two steps: (1) oxidative deamination to the a-keto acid (2-keto-4-(methylthiol)butyric acid) and (2) transamination of an amino group from glutamate. Most animals except primates readily convert d-methionine into l-methionine, and this is true for pigs. Although an early study indicated that the d-form was used less effectively than the l-form by very young pigs (Kim and Bayley, 1983), later studies indicated that dl-methionine and l-methionine were nutritionally equivalent (Reifsnyder et al., 1984; Chung and Baker, 1992a). The results of Chung and Baker (1992a) show equal utilization of d-, l- and dl-methionine by 10-kg pigs (Fig. 8.4). Commercial feed-grade methionine is in the dl-form.

In contrast to methionine, the d-isomer of cysteine does not have bioactivity. Apparently, there is no metabolic pathway from d-cysteine to the a-keto analogue of cysteine and therefore neither d-cysteine nor d-cystine has biological activity (Baker, 1994; Lewis and Baker, 1995).

Basai 0.25 0.50 0.50 0.50 0.57 g kg-"1 g kg-"1 g kg-"1 g kg-"1 g kg-"1 l-Met l-Met d-Met dl-Met dl-HMB

Fig. 8.4. Average daily gain of 10-kg pigs fed a basal diet (containing 1.9 g kg 1 methionine and 10 g kg 1 cystine) supplemented with various sources of methionine (Chung and Baker, 1992a). The amount of 0.57 g kg 1 HMB (2-hydroxy-4-(methylthiol)butyric acid, also known as methionine hydroxy analogue) is equimolar to 0.5 g kg 1 methionine. Bars without a common letter (a, b or c) differ (P< 0.05).

Basai 0.25 0.50 0.50 0.50 0.57 g kg-"1 g kg-"1 g kg-"1 g kg-"1 g kg-"1 l-Met l-Met d-Met dl-Met dl-HMB

Fig. 8.4. Average daily gain of 10-kg pigs fed a basal diet (containing 1.9 g kg 1 methionine and 10 g kg 1 cystine) supplemented with various sources of methionine (Chung and Baker, 1992a). The amount of 0.57 g kg 1 HMB (2-hydroxy-4-(methylthiol)butyric acid, also known as methionine hydroxy analogue) is equimolar to 0.5 g kg 1 methionine. Bars without a common letter (a, b or c) differ (P< 0.05).

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