Estimation and energy implications of flux of hexose through glycogen in fermentations of sugars in vitro with mixed ruminal microbes from dairy cows

Authors: Hall, Mary Beth; WHITE, ROBIN - Virginia Tech

Submitted to: EAAP International Symposium on Energy and Protein Metabolism and Nutrition
Publication Type: Abstract Only
Publication Acceptance Date: 5/20/2022
Publication Date: 9/12/2022
Citation: Hall, M., White, R.R. 2022. Estimation and energy implications of flux of hexose through glycogen in fermentations of sugars in vitro with mixed ruminal microbes from dairy cows. EAAP International Symposium on Energy and Protein Metabolism and Nutrition. 13(2022):262-263.

Interpretive Summary:

Technical Abstract: Glycogen is a storage polysaccharide produced from a variety of carbohydrates by ruminal protozoa and bacteria. Simultaneously synthesized, degraded, and utilized within microbes, it is challenging to quantify total hexose flux through glycogen. Glycogen synthesis diverts ATP from cell growth, requiring one ATP per hexose added. Consequently, prediction of microbial production to meet nutrient demands can be affected by glycogen production. The objective of this study was to determine the flux of hexose through the glycogen pool in mixed ruminal microbes in vitro. In vitro fermentations were performed using rumen inoculum from ruminally cannulated lactating Holstein cows maintained under protocols approved by the University of Wisconsin Animal Care and Use Committee. For two weeks before inoculum collection, diets contained 1% of dry matter of sugars used in the fermentations. Hall (2017) gives details of fermentations and sample handling. Six fermentations performed evaluating sugars and N supplementation used 3 g sugar/L, Goering and Van Soest medium, and additions of urea or tryptone, a peptide source, to vary the N concentration and type. Fermentations had nominal N g/L of 0.30 and 0.45, and glucose included a 0.15 N g/L (no basal tryptone addition) treatment. All fermentations included glucose at the lowest N addition. Fermentations were 5 h for glucose and 6 h for lactose and sucrose with hourly destructive sampling. Residual substrate, glycogen, organic acids, and microbial N were measured, and gas determined stoichiometrically from organic acids. All measures were expressed as milligrams of carbon (C) for analysis. Hexose flux through glycogen for each substrate in each treatment in each fermentation was calculated using the FME package of R statistical software. Individual rate constants were derived for each fermentation replicate by fitting the parameters to the available data using the deSolve package. Simultaneous equations were solved to describe entry at each hour of substrate and glycogen C into product pools, providing total C flux for each pool. The statistical model used with the MIXED procedure of SAS included carbohydrate, nonprotein N g/L, peptides g/L, and two-way interactions with carbohydrate; study and fermentation run within study were random variables. Values are reported as hexose umol to relate them to ATP costs. For all measures, lactose differed (P<0.01) from glucose and sucrose which did not differ from each other (P>0.05). Substrate hexose of 281, 408, and 440 umol was consumed for lactose, glucose, and sucrose, respectively. Detected maximum glycogen was 1 umol hexose for lactose, and 106 and 116 umol hexose for glucose and sucrose. Isotrichid protozoa which were present in the inoculum were reported to not convert lactose to glycogen (Oxford, 1951) which could give partial explanation to the reduced glycogen production with lactose. Total fluxes of hexose through glycogen were 55% (224 umol) and 62% (265 umol) of the total substrate consumed for glucose and sucrose, whereas directly measured glycogen maxima accounted for approximately 25% of consumed substrate. Similarly, yield of microbial glycogen was approximately 25% by weight of water-soluble sugars from hay fed to sheep (Oxford, 1951). This suggests potential for in vitro flux values to also apply in vivo. Lactose had a flux of hexose through glycogen of 32 umol. Unlike hexose flux, N source affected detected glycogen maxima, possibly via hexose diverted for microbial growth. Moles of hexose incorporated into glycogen equal moles of ATP used for glycogen synthesis. This reduces ATP available for microbial growth. It is of note that, although butterfat responses are relatively common when increasing sucrose in diets, milk protein values often stay the same or decline. With estimates of 8.03 mol of organic acids/kg ruminally ferment