Altered partition of threonine metabolism in pigs by protein-free feeding or starvation
1991
Ballevre, O. | Houlier, M.L. | Prugnaud, J. | Bayle, G. | Bercovici, D. | Seve, B. | Arnal, M.
Kinetic aspects of threonine (Thr) metabolism were examined in growing pigs fed a well-balanced diet (C), an isocaloric protein-free diet (PF), or starved (S) for 48 h. With the use of continuous simultaneous infusion of L-[1-13C]Thr, [1-14C]sarcosine, and 2-[1-14C]ketobutyrate (KB) for 10 h, estimates were made of rates of Thr incorporated into protein (S), released from body proteins (B), and oxidized through the catabolic pathways of L-Thr 3-dehydrogenase (TDG) and threonine dehydratase (TDH). In the C group S was 185, B was 138, Thr disposal to glycine (DR(Thr-Gly)) was 47, and Thr disposal to KB (DR(Thr-KB)) was 7 micromole.h-1.kg-1. Consequently, Thr balance was +48 micromole.h-1.kg-1. In the PF-fed pigs, S, B, DR(Thr-Gly), and DR(Thr-KB) were significantly reduced by 38, 15, 74, and 75%, respectively. In the S group, S, B, and DR(Thr-Gly) were significantly reduced by 47, 17, and 55%, respectively, but DR(Thr-KB) was similar to the C group. DR(Thr-Gly) in all groups was highly correlated with TDG enzyme activity measured in liver homogenates. By contrast with in vivo results, TDH enzyme activity was increased by 88% (P greater than 0.05) in the S group and decreased by 27% (not significant) in the PF group compared with the C group. The TDH pathway accounted for 13, 12, and 27% of total Thr oxidation in the C, PF, and S groups, respectively. These results suggest that Thr conservation in protein-depleted states (PF and S groups) occurred mainly by a decrease of Thr oxidation and that the partition through these pathways was only altered when energy was completely withdrawn.
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