The mode of regulation of pyruvate dehydrogenase of lactating rat mammary gland. Effects of starvation and insulin

Abstract

1. The ‘initial activity’ of the pyruvate dehydrogenase enzyme complex in whole tissue or mitochondrial extracts of lactating rat mammary glands was greatly decreased by 24 or 48h starvation of the rats. Injection of insulin and glucose into starved rats 60min before removal of the glands abolished this difference in ‘initial activities’. 2. The ‘total activity’ of the enzyme complex in such extracts was revealed by incubation in the presence of free Mg2+ and Ca2+ ions (more than 10 and 0.1mm respectively) and a crude preparation of pig heart pyruvate dehydrogenase phosphatase. Starvation did not alter this ‘total activity’. It is assumed that the decline in ‘initial activity’ of the enzyme complex derived from the glands of starved animals was due to increased phosphorylation of its α-subunit by intrinsic pyruvate dehydrogenase kinase. 3. Starvation led to an increase in intrinsic pyruvate dehydrogenase kinase activity in both whole tissue and mitochondrial extracts. Injection of insulin into starved animals 30min before removal of the lactating mammary glands abolished the increase in pyruvate dehydrogenase kinase activity in whole-tissue extracts. 4. Pyruvate (1mm) prevented ATP-induced inactivation of the enzyme complex in mitochondrial extracts from glands of fed animals. In similar extracts from starved animals pyruvate was ineffective. 5. Starvation led to a decline in activity of pyruvate dehydrogenase phosphatase in mitochondrial extracts, but not in whole-tissue extracts. 6. These changes in activity of the intrinsic kinase and phosphatase of the pyruvate dehydrogenase complex of lactating rat mammary gland are not explicable by current theories of regulation of the complex.

Integration of metabolism in tissues of the lactating rat

“…Thus acini from starved (24 h) lactating rats have a decreased rate of glucose utilization and the proportion of glucose utilized (expressed as Ca units) which accumulates in the medium as lactate and pyruvate is 64% compared to 16% with acini from fed lactating rats [45]. The experiments in vivo and in vitro suggest that glucose metabolism in starvation is inhibited at the stage of pyruvate oxidation and this is confirmed by the finding that pyruvate dehydrogenase in glands from starved rats is inactivated [46,47]. The rate of lactose synthesis in vivo as measured with [U-'4C]glucose is very sensitive to food withdrawal and the decreased rate occurs without any change in the activity of galactosyltransferase…”

“…The percentage of active enzyme is decreased to <IO% without any change in the total activity and this inactivation is reversed by administration of insulin [57]. Starvation is also associated with marked inactivation of mammary gland pyruvate dehydrogenase in vivo [46,47] and this can be reversed by administration of glucose and insulin [47].…”

Regulation of kinase reactions in pig heart pyruvate dehydrogenase complex

“…that inhibition of the pyruvate dehydrogenase kinase reaction(s) by pyrtuvate is influenced by nutritional ys and infactors, being impaired by starvation in the rat al section. Baxter & Coore, 1978 Fig. 5 )+.…”

Absent pyruvate inhibition of pyruvate dehydrogenase kinase in lactating rat mammary gland following various treatments

“…We have reported [5,6] that in lactating rat mammary gland 24 h starvation leads to a change m the behaviour of PDH kinase which persists m whole tissue or mrtochondnal extracts and is unlikely to be due to changes m relative concentrations of effector pairs (ATP/ADP, acetyl CoA/CoA and NADH/NAD) shown to operate on the analogous enzyme in other trssues [ 1, 7,8] . The PDH kinase from lactating mammary gland of starved rats 1s more active than that from glands of fed animals under apparently identical condrtions m vrtro and loses the property of being inhibited by pyruvate.…”

“…Earher data [lo] showed large increases m phosphorylatron of mammary gland PDH under these conditions. Since both drabetes and Sources of rats and chemicals were the same as m [4,6,10] with the followmg additions. Cyclohexrmide and puromycm (drhydrochlorrde) were obtamed from Sigma London, Norbiton Station Yard, Kingstonupon-Thames, Surrey KT2 7BH.…”