Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep: efficiency of utilization of acetate.

A previously described mathematical model that simulates the metabolism of absorbed nutrients was used to examine factors influencing the efficiency of utilization of acetate in a sheep weighing 25 kg. A significant component of the model was a function representing substrate cycles that degraded ATP when its concentration reached twice initial levels. Acetate utilization was simulated when up to 2 g mol/d were added to four diets (representing forages and concentrates) that supplied varying proportions of absorbed volatile fatty acids, glucose and protein. The basal diets provided either 4.375 (near energy balance) or 8.75 MJ/d absorbed energy. The predicted net efficiency of utilization (kf) of the energy in added acetate was high (0.58-0.70) for all combinations of absorbed nutrients at the low level of energy absorption, whereas at the higher level, it ranged from 0.16 (forage) to 0.49 (high protein concentrate). Low kf values were always associated with a considerable flux of ATP through the degradation pathway. The increase in ATP concentration that caused this pathway to operate was linked to an inhibition in the utilization of acetyl-CoA for fatty acid synthesis and an increase in its oxidation. The simulated addition of exogenous NADPH or its precursors (particularly glucose and propionate) to these diets repartitioned acetyl-CoA flux towards fatty acid synthesis instead of oxidation, decreased the flux of ATP through the degradation pathway and increased the kf of added acetate. Although a negative relationship was predicted between kf of added acetate and NADPH production for diets with increasing protein content, kf still depended on ATP flux through the degradation pathway. Addition of glucose to the high protein diets decreased this flux by decreasing acetyl-CoA oxidation and increasing fatty acid synthesis. The predictions suggest that the efficiency of acetate utilization in ruminants may be influenced by NADPH availability when a considerable amount of absorbed energy is derived from acetate. Increased absorption of glucose or propionate enhanced NADPH production, but the major effect on acetate utilization of increasing dietary protein was not through NADPH production.