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This work investigated the metabolic basis for the variability of carcass and i.m. adiposity in cattle. Our hypothesis was that the comparison of extreme breeds for adiposity might allow for the identification of some metabolic pathways determinant for carcass and i.m. adiposity. Thus, 23- to 28-mo-old steers of 3 breeds, 2 with high [Angus or Japanese Black x Angus (J. Black cross)] and 1 with low (Limousin) i.m. and carcass adiposity, were used to measure activities or mRNA levels, or both, of enzymes involved in de novo lipogenesis [acetyl-coA carboxylase, fatty acid synthase (FAS), glucose-6-phosphate dehydrogenase (G6PDH), malic enzyme], circulating triacylglycerol (TAG) uptake (lipoprotein lipase), and fatty acid esterification (glycerol-3-phosphate dehydrogenase), as well as the mRNA level of leptin, an adiposity-related factor. In a first study, enzyme activities were assayed in the s.c. adipose tissue (AT), the oxidative rectus abdominis, and the glycolytic semitendinosus muscles from steers finished for 6 mo. Compared with Angus or J. Black cross, Limousin steers had a 27% less (P = 0.003) rib fat thickhess, and 23 and 29% less (P [less than or equal to] 0.02) FAS and G6PDH activities in s.c. AT. In rectus abdominis and semitendinosus, the 75% less (P < 0.001) TAG content was concomitant with 50% less (P < 0.001) G6PDH activity. In a second study, enzyme activities plus mRNA levels were assayed in an oxido-glycolytic muscle, the longissimus thoracis (LT), in the i.m. AT dissected from LT, and in s.c. AT from the same Limousin steers and from Angus steers finished for 10 mo. Compared with Angus, the 50% less (P < 0.001) rib fat thickness in Limousin contrasted with the 1.1- to 5.8-fold greater (P [less than or equal to] 0.02) mRNA levels or activities, or both, of acetyl-coA carboxylase, G6PDH, lipoprotein lipase, and glycerol-3-phosphate dehydrogenase in s.c. AT. Conversely, the 90% less (P < 0.001) TAG content in Limousin LT was concomitant to the 79 and 83% less (P [less than or equal to] 0.002) G6PDH activity and leptin mRNA level. Such differences could arise from a greater number of adipocytes in LT from Angus steers because no difference was found between Limousin and Angus for G6PDH activity and leptin mRNA in i.m. AT. We conclude that FAS and G6PDH in s.c. AT could be involved in differences in carcass adiposity, but this relationship disappeared when the fatness increased strongly. Leptin and G6PDH are related to the expression of marbling whatever the body condition and thus could be relevant indicators of marbling in beef cattle. Key words: adipose tissue, bovine, leptin, lipogenic activity, muscle

Marbling is an important criterion for beef quality grading in many countries. The purpose of the current study was to utilize the natural genetic variation to identify major metabolic indicators of marbling in cattle differing in genotypes. Rectus abdominis (RA, oxidative), semitendinosus (glycolytic), and longissimus thoracis (LT, oxido-glycolytic) muscles were taken from steers of different genotypes that expressed high [Angus, n = 16; and crossbred (Angus x Japanese Black), n = 10] or low (Limousin, n = 12) levels of marbling in their meat. Muscles from Angus and crossbred steers were characterized, as expected, by a greater triacylglycerol (TAG) content (P < 0.001) and also by greater protein contents of fatty acid-binding protein specific for heart and muscles (H-FABP; P < 0.001 for RA and P < 0.05 for LT muscle) or for adipocytes (A-FABP; P < 0.001 for RA and LT muscles). Moreover, oxidative enzyme activities ([beta]-hydroxyacyl-CoA dehydrogenase, citrate synthase, isocitrate dehydrogenase, cytochrome-c oxidase) were greater (P < 0.01 to 0.001) in the 3 muscles studied, whereas glycolytic enzyme activities (phosphofructokinase and lactate dehydrogenase) were lower (P < 0.001) in RA muscle in Angus and crossbred steers compared with Limousin steers. Significant correlations were observed between TAG content and H- and A-FABP protein contents, and oxidative (r [less than or eqaul to] +0.55, P < 0.001) or glycolytic enzyme activities (r [less than or equal] -0.47, P < 0.001), when the 3 genotypes and muscles studied were considered as a whole. In addition, A-FABP protein content and some oxidative enzyme activities were significantly correlated with TAG content independently of the genotype and muscle effects. In conclusion, A-FABP protein content, as well as oxidative enzyme activities, may be used as indicators of the ability of steers from extreme genotypes to deposit intramuscular fat. Key words: triacylglycerol, marbling, fatty acid-binding protein, muscle metabolism, genotype, steer