Effects of sex, weight, diet and hCG administration on levels of skatole and indole in the liver and hepatic activities of cytochromes P4502E1 and P4502A6 in pigs
Introduction
Boar taint, an offensive odour in pork products, is one main factor limiting the production of meat from entire male pigs. Boar taint occurs mainly due to the accumulation of skatole and androstenone in porcine adipose tissue. Indole may also contribute to boar taint although to a lesser extent. Skatole (3-methylindole) is a metabolite of tryptophan in the large intestine of pigs of all genders, but only entire males accumulate high skatole levels in adipose tissue, probably due to sex-dependent differences in the potential of pigs to metabolise skatole. Skatole is eliminated from the porcine body in the form of several metabolites (Diaz et al., 1999, Diaz and Squires, 2003). Cytochromes P4502E1 (CYP2E1) and P4502A6 (CYP2A6) are the key enzymes involved in hepatic skatole metabolism (Babol et al., 1998, Diaz and Squires, 2000, Squires and Lundström, 1997). High skatole levels in porcine adipose tissue are associated with low activities of those enzymes (Zamaratskaia et al., 2005a). Additionally, skatole levels considerably depend on nutritional factors. The addition of raw potato starch (RPS) markedly reduced skatole levels in fat and plasma from castrated (Claus, Losel, Lacorn, Mentschel, & Schenkel, 2003) and entire male pigs (Zamaratskaia, Babol, Andersson, Andersson, & Lundström, 2005b). Claus et al. (2003) explained the effect of RPS on skatole levels by the increased butyrate production as a result of bacterial fermentation in the large intestine. Butyrate inhibited colonocyte apoptosis (Mentschel & Claus, 2003) and thus reduced the availability of cell debris, which is a substrate for skatole synthesis.
Indole, like skatole, is a product of tryptophan metabolism in the large intestine of pigs. The metabolic pathway(s) of indole has not been extensively studied and little is known about enzymes involved in indole metabolism. There is, however, evidence that indole metabolism in the liver is mediated by cytochromes CYP2E1 and CYP2A6 (Banoglu et al., 2001, Gillam et al., 2000).
Androstenone (5-α-androst-16-en-3-one) is a steroid produced by the testis of mature male pigs (Bonneau, 1982). Androstenone was shown to block the expression of CYP2E1 in vitro (Doran et al., 2002a, Tambyrajah et al., 2004). However, as yet there is no direct evidence showing that increased levels of androstenone suppress the activities of CYP2E1 in vivo, and whether such suppression, if it occurs, might cause increased skatole levels. Treatment with human chorionic gonadotropin (hCG) stimulates androstenone secretion by testicular Leydig cells (Andresen, 1975, Bonneau et al., 1982, Carlström et al., 1975, Malmfors et al., 1976). Hormonal stimulation might be a way to provoke pigs to increase androgen secretion, which in turn might suppress the expression of CYP2E1 and subsequently reduce its activity. Diet does not affect androstenone production unless the level of nutrition affects the sexual maturity.
The first aim of the present work was to evaluate the potential sex-related differences in the activities of CYP2E1 and CYP2A6 in porcine liver microsomes. The second aim was to investigate the effect of hCG stimulation on the activities of CYP2E1 and CYP2A6. The third aim was to study the effect of dietary supplement of RPS on the levels of skatole and indole in the liver and the activities of CYP2E1 and CYP2A6.
This study represents a further step in the effort to understand the sex-specific differences in skatole levels and the role of testicular steroid androstenone in the regulation of skatole metabolism.
Section snippets
Animals
The study was a part of a Swedish project on raising entire male pigs and the effects of raising system and slaughter weight on boar taint and aggression level. A total of 27 female pigs and 54 entire male pigs of a crossbred (Swedish Yorkshire dams × Landrace sires) were included in the study. Animals were raised either in single-sex or mixed pens with 7 or 9 pigs in each. All pigs were fed the same commercial diet according to the standard feeding regimen for finishing pigs in Sweden
Effect of sex
Female and entire male pigs showed similar CYP2E1 activity at 90 kg (p = 0.70) (Table 2), whereas the activity of CYP2A6 at this live weight was higher in female pigs (p = 0.04). At a live weight of 115 kg, the activity of CYP2E1 was higher in female pigs compared to that of entire males (p = 0.02). No differences in CYP2A6 activity were found between female and male pigs at 115 kg LW (p = 0.68).
Skatole levels in the liver were higher in male pigs than in females at both 90 kg LW (p = 0.02) and at 115 kg LW (
Discussion
In this study, the concentrations of skatole and indole in porcine liver were measured by an HPLC method developed and validated in our laboratories. The proposed method is rapid and reproducible and is therefore suitable for routine analysis of skatole and indole in liver samples. Additionally, we used methanol as extraction solvent, which is less toxic than other solvents, such as acetonitrile or chloroform. In this study, the levels of skatole and indole in the liver were studied in relation
Conclusion
The activity of CYP2A6 was higher in female pigs at 90 kg compared with male pigs. At 115 kg LW, the activity of CYP2A6 did not differ between sexes, whereas the activity of CYP2E1 was higher in female pigs. Skatole levels in the liver were higher in male pigs at both LW, and indole levels were higher in male pigs at 115 kg LW. It is possible that these differences in enzyme activities and skatole levels in the liver may contribute to the sex-related prevalence of boar taint in pork products.
Acknowledgements
This study was funded by The Swedish Board of Agriculture. Lyckeby Culinar AB is gratefully acknowledged for providing raw potato starch. The authors thank the staff at Lövsta Research Station involved in raising the pigs, and especially Ulla Schmidt for the help with the blood sampling.
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