Introduction
Adipocyte Determination and Differentiation Factor-1
The ADD1, also known as, sterol regulatory element binding protein-1 (SREBP1) in human and mouse studies, is a transcription factor believed to play a role in encoding enzymes of lipid biosynthesis (Wang, et al., 1994; Shimomura et al., 1998), and may also be involved in the control of plasma cholesterol levels (Yokoyama et al, 1993). The ADD1 has been reported to increase the activity of peroxisome proliferator activated Receptor-γ (PPARγ), another transcription factor involved in adipocyte differentiation (Kim et al., 1998), and Boizard et al. (1998) demonstrated that ADD1 is involved in the over expression of the fatty-acid synthase gene (FAS) in obese mice. Along the same lines, Ding et al. (1999) reported that the expression of ADD1 is positively associated with porcine adipocyte differentiation both in vivo and invitro. Shimomura et al. (1998) demonstrated a decrease in fat deposits in transgenic mice that over expressed SREBP1. Given the physiological effects of this gene in other mammalian species, ADD1 may also control backfat deposition in the pig, and may correlate with variation in meat quality traits.
Pyruvate Dehydrogenase E1-alpha
The pyruvate dehydrogenase (PDH) complex contains multiple copies of three enzymes: E1, E2 and E3 (Ho et al., 1989). The E1-alpha subunit plays a key role in the function of the PDH complex since it contains the E1 active site, which may be thought of as the enzyme's "on/off" switch. The E1-alpha subunit functions specifically to catalyze the decarboxylation of pyruvate into acetyl-CoA (Reed, 1974). A deficiency in PDH is one of the most commonly defined genetic defects of mitochondrial energy metabolism in humans (Robinson et al., 1987). The PDH complex effects energy metabolism in the cell as it is a rate-limiting enzyme connecting glycolysis with the tricarboxylic acid (TCA) cycle (Lissens et al, 2000). Thus, the PDH complex plays an essential role in carbohydrate metabolism. Ward et al. (1982) reported that activation of PDH regulates the use of glycogen and glucose during exercise in humans, while Hagg et al. (1976) found that PDH activity is increased in rats that were exercised. Peters et al. (1998) reported that carbohydrate metabolism is decreased when PDH is down-regulated.