Weaning Elicits a Distinct Increase in Plasma Cysteine Concentration in Pigs

Abstract Weaning is a stressful event in pigs that is associated with low and variable feed intake, poor nutrient utilization, and high feed costs. The sulfur amino acids (SAA) methionine and cysteine have key roles in pig production, but how SAA metabolism, and thus SAA requirements, is affected by weaning is poorly understood. The objective of this study was to determine weaning-induced changes in SAA metabolism. Pigs (initial BW = 7,233 ± 683 g) were either weaned at 21 d age and euthanized at 23 d age (n = 4) or remained with the sow and euthanized at 23 d age (n = 4). Weaned pigs were fasted but had free access to water. At euthanasia, blood and liver tissue were collected. Plasma amino acids and amino thiols were analyzed by HPLC following pre-column derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate and 4-fluoro-7-aminosulfonylbenzofurazan, respectively. The protein abundance of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CTH), enzymes needed for methionine transsulfuration to cysteine, in liver were determined by Western blot. Total plasma non-essential amino acid (NEAA) concentrations declined in weaned compared with unweaned pigs (2,664 versus 4,290 ± 211 µmol/L, P < 0.01). Among NEAA, however, only plasma cysteine concentration increased in weaned compared with unweaned pigs (total cysteine, 253 versus 173 ± 16 µmol/L, P < 0.05; cystine, 43 versus 23 ± 5 µmol/L, P < 0.05). Plasma methionine tended to be less in weaned pigs (53 versus 65 ± 4 µmol/L, P = 0.075), whereas plasma taurine was less in weaned pigs (64 versus 128 ± 10 µmol/L, P < 0.01). The abundance of CBS and CTH protein in liver was not different between groups. Elevated plasma cysteine concentrations after weaning in pigs is likely due to decreased cysteine oxidation to taurine rather than greater methionine transsulfuration to cysteine.