Precursors of hexoneogenesis within the human mammary gland

Abstract

The human mammary gland is capable of de novo synthesis of glucose and galactose (hexoneogenesis); however, the carbon source is incompletely understood. In this study, we investigated the role of acetate, glutamine, lactate and glycerol as potential carbon sources for hexoneogenesis. Healthy breastfeeding women were studied following a 24-h fast on two occasions separated by 1-3 wk. Five women were infused with [U-13 C]lactate or [1,2-13 C2]glutamine and five women with [U-13 C]glycerol or [1,2-13 C2]acetate. Enrichments of 13 C in plasma and milk substrates were analyzed using GC-MS. Infusion of labeled lactate, glycerol, glutamine, and acetate resulted in plasma glucose being 22.0 Ϯ 3.7, 11.2 Ϯ 1.0, 2.5 Ϯ 0.5, and 1.3 Ϯ 0.2% labeled, respectively. Lactate, glutamine, or acetate did not contribute to milk glucose or galactose (0 -2%). In milk, 13 C-free glycerol enrichment was one-fourth that in plasma but free glycerol concentration in milk was fourfold higher than in plasma. Using [U-13 C]glycerol and by accounting for tracer dilution, glycerol alone contributed to 10 Ϯ 2 and 69 Ϯ 11% of the hexoneogenesis of milk glucose and galactose, respectively. During [U-13 C]glycerol infusion, the ratio of M3 enrichment on 4 -6 carbons/M3 on 1-3 carbons of galactose was higher (P Ͻ 0.05, 1.22 Ϯ 0.05) than those of glucose in plasma (1.05 Ϯ 0.03) and milk (1.07 Ϯ 0.02). Reanalysis of samples from a previous study involving [U-13 C]glucose infusion alone suggested labeling a portion of galactose consistent with pentose phosphate pathway (PPP) activity. We conclude that, although lactate contributed significantly to gluconeogenesis, glycerol alone provides the vast majority of substrate for hexoneogenesis. The relative contribution of the PPP vs. the reversal Embden-Meyerhof pathway to hexoneogenesis within the human mammary gland remains to be determined. galactose; lactose synthesis; gluconeogenesis; pentose phosphate pathway; stable isotopes GC-MS WE PREVIOUSLY DEMONSTRATED (26) that plasma glucose is the predominant but not the exclusive carbon source of milk lactose (80% during the fed state and 60% following 24 h of fasting). Thus, the mammary gland is capable of de novo synthesis of glucose and galactose, a process we termed hexoneogenesis. During feeding, hexoneogenesis contributes 20% of the carbon in lactose (10 and 30% of glucose and galactose, respectively) (20,26,27). Over a 14-to 42-h fast, hexoneogenesis increases to account for ϳ40% of lactose production (ϳ30% of glucose and ϳ50% of galactose) (18,20,21,26,27). However, the precursor carbon source for hexoneogenesis remains largely unknown.We hypothesize that those traditional gluconeogenic substrates for hepatic glucose production will also serve as precursors for hexoneogenesis. Lactate, a predominant gluconeogenic precursor in humans, accounts for 50 -70% of the total gluconeogenic flux in normal humans (3,12). Another significant precursor of glucose synthesis (particularly in the kidney) is glutamine, which together with glutamic acid are the most ab...

Biosynthesis of milk fat, protein, and lactose: roles of transcriptional and posttranscriptional regulation

“…In a recently published study performed in humans, glycerol contributed 10% and 69% of de novo synthesized glucose and galactose, respectively, for lactose synthesis (148). In 1958, it was demonstrated by an elegant unilateral glycerol-1,3 C 14 -labeled injection experiment that glycerol contributes significantly to the galactose produced in the mammary gland of dairy cows, in addition to contributing to fatty acid and casein syntheses (242).…”

Measurements of Gluconeogenesis and Glycogenolysis: A Methodological Review

“…We have used a similar approach during entral absorption. By using small frequent feeding over a 4–5-h period to achieve near steady-state enrichment, we believe we have been able to obtain reasonable estimates of gluconeogenesis that are reproducible and mimic the glucose homeostasis achieved after a bolus meal ( 26 , 27 ).…”

“…However, because of the factors mentioned above, there is no gold standard (and most likely will never be) by which to judge the relative value of one method over another. We have used and published studies using most of the isotopic methods outlined ( 12 , 26 , 27 , 31 – 36 ) and therefore hope we stand in a good position to provide a reasonable evaluation of each. We will acknowledge our potential bias as a result of our experience, but we attempt to present a fair and valid methodological evaluation.…”

“…Mass isotopomer analysis of [U- 13 C]glucose was proposed as a suitable alternative to the isotope methods discussed above ( 57 ). This method allowed simultaneous measurements of total glucose turnover (M+6), fractional gluconeogenesis (MIDA calculation of primarily the M+1, M+2, and M+3 masses of glucose), estimates of the recycling of glucose (Cori cycle), and perhaps hexomonophosphate shunt activity using the M+4 masses ( 27 ). Depending on the exact MIDA analysis used, gluconeogenic calculations included various correction factors to account for precursor pool estimation.…”

“…Using [U- 13 C]glucose in lactating women, we found evidence of hexomonophosphate shunt activity in determining the carbon source for milk galactose. Additional studies will be required to evaluate the relative importance of the hexomonophosphate shunt under a number of metabolic conditions ( 27 ).…”