Journal of Lipid Research, Vol 17, 467-477, Copyright © 1976 by Lipid Research, Inc.
The absolute rate of fatty acid synthesis by mammary gland slices from lactating rats
JC Bartley and S Abraham
Mammary gland utilizes a combination of substrates in vivo. To determine if
and how the substrates presented to the gland alter milk fat synthesis,
slices of mammary gland of lactating rats were incubated with combinations
of glucose, L-lactate, and pyruvate. Uptakes and conversions of uniformly
and specifically labeled substrates to CO2 and fatty acid were measured.
The absolute rate of fatty acid synthesis was measured by incoporation of
tritium from tritiated water into fatty acid. The extent, but not the type,
of fatty acids synthesized was affected by the substrates utilized. Glucose
stimulated uptake and conversion to fatty acid of l-lactate and, to a
smaller extent, of pyruvate. Analysis of CO2 and fatty acid yields revealed
that (a) the major stimulatory effect of glucose was on the conversion of
acetyl coenyzme A from other substrates to fatty acid; (b) the rate of
fatty acid synthesis paralleled the activity of the hexose monophosphate
pathway up to a point. A likely source of the NADPH required beyond this
point is provided by oxidation of malate to pyruvate. Reduction of NADH of
oxaloacetate, produced during citrate cleavage, would yield malate. Hence,
maximal synthesis would require production of NADPH and NADH, a condition
met when glucose and L-lactate were substrates. Decreased fatty acids
synthesis in pyruvate's presence supports this suggested requirement for
cytosolic NADH in rat mammary gland.
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