Transsulphuration occurs in the liver and other visceral organs (Radcliffe and Egan, 1978), but not in skeletal muscle (Radcliffe and Egan, 1974). Transsulphuration can be measured using isotope-labelled Met and Cys with constant infusion techniques. Some measurements based on the plasma kinetics are listed in Table 17.5. These results from various sources show a similar trend that about 0.04-0.22 Cys is derived from transsulphuration, which uses less than 0.1 Met. Since these measurements refer
3C compounds, SO4 , OO2
HomoCys Ts|*- Ser
SO4 , taurine
Fig. 17.1. The catabolic pathways of methionine. Abbreviations: Ado-Met, adenosylmethionine; Cys, cysteine; HomoCys, homocysteine; Met, methionine; RM, remethylation; Ser, serine; TA, transamination; TS, transsulphuration.
Table 17.5. Met flux and the rate of transsulphuration (as proportions of Met or Cys flux) in sheep. Met flux was recalculated from the original flux values and the corresponding liveweight of the animals.
Refe re n oes
Suffolk cross 0.70-1.73 30 kg
Merino wether 0.50-1.96 30 kg
Pregnant Merino ewes 0.85-1.63 37 kg
Romney wether 0.69-1.99 29-33 kg
0.02-0.12 0.05-0.19 Pisulewski and Buttery
0.03-0.09 0.10-0.24 Egan et al. (1984) 0.03-0.05 0.06-0.22 Williams et al. (1988) 0.08-0.10 0.06-0.14 Lee et al. (1995)
to the whole-body metabolism, the actual distribution of synthesized Cys to the skin needs to be calculated using the skin : whole-body blood flow ratio. Blood flow to the whole skin of Romney sheep accounted for about 0.06 of the cardiac output (Harris et ai, 1989), therefore, Cys synthesis de novo from the systemic distribution is unlikely to make a significant contribution to wool growth.
There is, however, evidence that transsulphuration occurs within the skin (including wool follicles). Downes et ai (1964) gave Merino sheep an intradermal injection of [35S]Met and found 0.9 of the 35S incorporated into the wool was present as Cys. With an Infusion of [35S]Met, a significant appearance of 35S label in the skin cystathionine (an intermediate in transsulphuration) pool and Cys pool was observed in Romney sheep (Harris et ai, 1997). The direct evidence is from experiments in vitro where inclusion of Met but not Cys in the medium enabled fibre elongation and follicle viability in follicles obtained from Merinos (Hynd and Nancarrow, 1996) or Angora goats (Souri et ai, 1996).
The issue is how important for wool growth is the quantity of Cys synthesized from transsulphuration in the skin or the follicles. Souri et ai (1996) found Met alone could produce a growth response approximately 0.8 that of Met plus Cys combined in cultured follicles. This probably represents the maximum rate of transsulphuration in the follicles since an inclusion of Cys will feedback on the activities of the enzymes Involved in the pathways. Harris et ai (1997) measured the net uptake of Cys by the skin of Romney sheep using arterio-venous techniques and estimated that the uptake of Cys accounted for only 0.11-0.28 of the Cys that was retained in wool, with the rest coming from local synthesis of Cys or other sources such as the tripep-tide glutathione. Although the estimates are semiquantitative, the indications are that local synthesis in the skin and follicle provide a substantial source of Cys for wool growth.
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