Transsulfuration, protein synthesis rate and follicle mRNA in the skin of young Merino lambs in response to infusions of methionine and serine

Methionine (Met) is usually the first limiting amino acid for sheep and sup plements of Met may increase production of wool and meat. The wool response may be due to an increased supply of cysteine (Cys) from transsulfuration (TS) of Met. Met is catabolized through homocysteine to form Cys when the S from Met is transferred to serine (Ser). We hypothesized that providing ad ditional Met would create a deficiency of Ser and that by simultaneously pr oviding Met and Ser, TS and wool growth could be increased more than by pro viding Met alone. The effects of i.v. infusions of Met and Ser to young Mer ino lambs on TS, fractional synthesis rate (FSR) of protein in skin, follic le mRNA and wool growth were examined. Following 4 d of constant i.v. infus ion of 3 g Met/d, or 10 g Ser/d or both, the isotope tracers: L-[3-C-13]Cys , L-[ring-d(5)]phenylalanine (Phe) and L-[2,3,3-d(3)]Ser were infused over 8 h to allow for measurements of irreversible loss rate (ILR), and TS in wh ole body and skin. Skin biopsies were taken for measurement of FSR. Wool gr owth rate was measured using autoradiography. An infusion of Met significan tly (P < 0.05) improved wool growth rate and increased skin FSR, Cys supply from TS and enhanced levels of follicle mRNA (from the K2.10 intermediate filament gene and three gene families encoding keratin associated proteins KAP1, KAP4 and KAP12). The extra Met lowered Ser ILR. The infusion of Ser d oubled Ser ILR in the body and increased skin FSR calculated using the Cys tracer in plasma (P < 0.05). However, there were no significant (P > 0.05) changes in TS, skin FSR calculated using the Phe and Ser tracers, follicle mRNA or wool growth rate as a result of Ser infusion. While there were tren ds towards increased TS and FSR with Ser infusion, the overall lack of sign ificant changes indicates a high capacity for the de novo synthesis of Ser.