Characterization of transport systems for cysteine, lysine, alanine, and leucine in wool follicles of sheep

¹ Discipline of Molecular and Biomedical Science, The University of Adelaide, Adelaide, South Australia, Australia 5001
² Discipline of Agricultural and Animal Science, The University of Adelaide, Roseworthy Campus, Roseworthy, South Australia, Australia, 5371
³ South Australian Research and Development Institute Livestock Systems, Roseworthy Campus, Roseworthy, South Australia, Australia, 5371

^* To whom correspondence should be addressed. E-mail: philip.hynd{at}adelaide.edu.au.

	Abstract

Aspects of the uptake of the AA cysteine, leucine, alanine and lysine into wool follicles were investigated using short-term culture of thin strips of sheep skin. Following verification of the veracity of the model system, the sites of uptake of the radiolabelled AA were shown to differ and to be consistent with their different roles in fiber production. Cysteine appeared in the zone of keratinization immediately distal to the follicle bulb. Lysine was incorporated into the germinative cells of the follicle bulb and the cells of the inner root sheath. Leucine and alanine were incorporated into the follicle bulb, inner root sheath, and keratinizing fiber. The incorporation of all AA into the dermal papilla was low. The relative rates of uptake of the AA into the wool follicle were: L-cysteine (100), L-leucine (5.5), L-alanine (2.5), L-lysine (0.8). Uptake of cysteine was saturable and followed Michaelis-Menten kinetics, suggesting a carrier-mediated system, with no diffusion. The majority (70%) of cysteine uptake into follicles was via a sodium-independent system that was not inhibited by -(methylamino)isobutyric acid (MeAIB) or 2-amino-2-norbonanecarboxylic acid (BCH), and therefore is not via the normal cysteine transport systems A, ASC, or L. Uptake of cysteine appeared to be via a low-affinity, high-capacity transport system which may be unique to the fiber-producing follicle. The majority of alanine transport had characteristics consistent with the functioning of System A (sodium-dependent, inhibited by MeAIB and low substrate affinity). Leucine uptake was inhibited by BCH but was sodium-dependent, suggesting a variant of system L operates in the follicle to transport leucine. Lysine uptake was consistent with the operation of the usual lysine-transporter system y+. Diets designed to maximize wool growth should provide AA profiles reflecting the relative rates of uptake demonstrated in this study. Investigations of possible polymorphisms in genes encoding AA transport proteins in follicles may reveal a source of genetic differences in wool growth potential between genotypes.

Key Words: amino acid, follicle, kinetics, sheep, wool