Model of metabolite flux within mammary gland of the lactating cow.

A balance model of milk synthesis was developed. The model balances carbon flow into and out of the gland and generates sufficient energy and reducing equivalents to meet synthetic requirements. Calculated uptakes of glucose, acetate, and betahydroxybutyrate were sufficient to meet gland requirements. Estimated uptakes of essential amino acids were less than output in milk and had to be adjusted to balance the model. Triacylglyceride fatty acid uptake from blood plasma was less than required for milk fat synthesis. A possible uptake of other plasma fatty acids was postulated to balance the model. Availability of glucose and oxidizable substrate suggested 42% of reducing equivalents for fat synthesis were generated in the pentose cycle and the remainder (58%) by isocitrate dehydrogenase. The balance model was used to formulate a dynamic model with equations to trace fates of isotopically labeled carbons from a variety of substrates. These were used to evaluate effects of changing nutrient availability and rate constants upon patterns of tracer distribution in products and to identify experimental data required to define rate constants uniquely in the model. Additional data are required to define equations for a number of key reactions that exhibit nonlinear kinetics in vivo.