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

Abstract

Aspects of the uptake of the AA Cys, Leu, Ala, and Lys into wool follicles were investigated using short-term culture of thin strips of sheep skin. Following verification of the reliability of the model system, the sites of uptake of the radiolabeled 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 Ala 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 as follows: L-Cys (100), L-Leu (5.5), L-Ala (2.5), and L-Lys (0.8). Uptake of Cys was saturable and followed Michaelis-Menten kinetics, suggesting a carrier-mediated system, with little or no diffusion. The majority (70%) of Cys uptake into follicles was via a Na-independent system that was not inhibited by alpha-(methyl-amino)isobutyric acid or 2-amino-2-norbonanecarboxylic acid and therefore is not via the normal Cys transport systems A, ASC, or L. Uptake of Cys appeared to be via a low-affinity, high-capacity transport system, which may be unique to the fiber-producing follicle. The majority of Ala transport had characteristics consistent with the functioning of system A (Na-dependent, inhibited by alpha-(methylamino)isobutyric acid, and low substrate affinity). Leucine uptake was inhibited by 2-amino-2-norbonanecarboxylic acid but was Na-dependent, suggesting that a variant of system L operates in the follicle to transport Leu. Lysine uptake was consistent with the operation of the usual Lys 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 among genotypes.