Met is also catabolized through the aminopropylation pathway, and provides the amino-propyl group for synthesis of spermidine and spermine (see Fig. 4.3). The synthesis costs equimolar amounts of Met for spermidine or 2 mol of Met for each mole of spermine. The sulphur and methyl group may be reincorporated into Met through the 5'-methyl-thioadenosine salvage pathway (Stipanuk, 1986). Polyamines (putreseine, spermidine and spermine) are required for optimal growth in all cells and their physiological functions are related to DNA, RNA and protein biosynthesis (Tabor and Tabor, 1984). Putrescine and spermidine are essential for cell proliferation and growth of the cultured wool follicles, with an optimal concentration of spermidine required for the greatest rate of fibre elongation (Hynd and Nancarrow, 1996). In addition, suppression of putrescine or spermidine de novo synthesis in vivo using the inhibitors of ornithine decarboxylase or of S-adenosyl-methionine decarboxylase reduces fibre length growth rate by 10-22%, and increases fibre diameter by 1-3 jxm and sulphur content in wool by 9% (Reis and Hynd, 1989; Hynd and Nancarrow, 1996). In contrast, intradermal injections of spermidine at doses of 1.38, 2.75 or 4.58 ¡xmol per injection at 8 h intervals for 7 days increased fibre length growth rate but not fibre diameter in Merino wethers (Liu et al., 2002). These results indicate that polyamines play a vital role in maintaining wool follicle function.
The use of Met for polyamine synthesis has not been quantified in sheep. In intact rats, 0.3 S-adenosylmethionine was catabo-lized through the aminopropylation pathway and the remaining 0.7 through the transsul-phuration pathway. A similar partition ratio was obtained from in vitro experiments with human cell lines (as reviewed by Stipanuk, 1986). Expressed as a proportion of polyamines, about 0.45 spermidine plus spermine was derived from de novo synthesis in rats (White and Bard6cz, 1999). The contribution of dietary polyamines to the body pool is low because of the low concentrations of polyamines in feeds, being about 0.0035%, 0.003% and 0.001% for putrescine, spermidine and spermine (Eliassen, 1982) and rapid degradation of polyamines in the rumen (Os et al., 1995). This conclusion is supported by a low abomasal recovery of most amines, for example 0.05-0.20 in cattle (Phuntsok et al., 1998). These data from various sources suggest that exogenous contributions to the large pool of polyamines in the whole body in ruminants would be limited. The de novo synthesis therefore, must be quantitatively important. The polyamine pool in the body appears to be much larger than that of Met, as their concentrations are substantially greater than Met, for example, 144-157 ¡xmol kg-1 wet tissue for spermidine vs. 31-38 ¡xmol kg 1 for Met (Liu et al., 2002). This suggests that considerable amounts of Met could be consumed for polyamine synthesis.
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