The estimation of purines in nucleic acids: a method applicable to materials with low concentration of nucleic acid

Section G. General. Nitrogen metabolism and the rumen

The fact that rumen micro-organisms have a modifying effect on the utilization of food nitrogen by ruminants has been recognized for very many years (references to early work are given in Hungate, 1966) but until about 1947, when this subject was reviewed by McNaught & Smith (1947), many of the processes involved were not well understood. Since that time a large amount of work has been done in this field and more recent reviews (Chalmers & Synge, 1954; Blackburn, 1965; Hungate, 1966; Waldo, 1968) present a much clearer picture. Most of the work covered by these authors will be mentioned only briefly in the present review and attention will be directed mainly to developments reported in the last few years. Few references to work published before about 1964 will be given as they may readily be found in later publications.

[The use of ribonucleic acid as a marker for measuring microbial protein yield in the rumen. 1. Chemical determination of ribonucleic acid]

The determination of ribonucleic acid (RNA) in the duodenal digesta and the rumen microbes was carried out colorimetrically with orcinol after extraction with hot NaCl solution, precipitation with phosphotungstic acid and alkaline hydrolysis. 96 +/- 1% of the RNA supplemented to the digesta content was recovered (y +/- s, n = 5). The methodic error (repetition accuracy) was 1.1%. The relation of RNA-N:N (mg/g) in the bacteria mass amounted to 93.8 +/- 3.99 (y +/- s(-y), n = 48) and to 16.1 +/- 0.8 (y +/- s(-y), n = 51) mg RNA/g DM in the duodenal content.

Nucleic acid metabolism in the ruminant. 2. Formation of microbial nucleic acids in the rumen in relation to the digestion of food nitrogen, and the fate of dietary nucleic acids

1. Concentrations of nucleic acid nitrogen (NA-N) and other nitrogenous constituents were estimated in samples of rumen fluid taken from calves and cows which were either given stall diets of varying nitrogen content or were allowed to graze pasture. Concentrations of NA-N ranged from 1.5 to 27.5 mg/100 g water.

2. Ratios of RNA: DNA in rumen fluid were similar to those in rumen bacteria and were not related to those in the diets. Pure nucleic acids added to the rumen were rapidly degraded. It was therefore concluded that NA-N in rumen fluid was largely of microbial origin and provided an index of total microbial nitrogen.

3. In most experiments, with an individual animal consuming diets of various nitrogen contents, NA-N formed a fairly constant percentage (8-15 for different animals) of the total non-ammonia nitrogen in rumen fluid. This suggested that nitrogen entering the rumen fluid limited microbial growth. Consumption of a diet containing extracted decorticated groundnut meal (diet B), however, led to lower values for this percentage than did the other diets. Diet B was also exceptional in leading to marked diurnal variations in NA-N concentrations in rumen fluid, suggesting a cyclic fluctuation in the size of the microbial population.

4. Comparison of NA-N:total nitrogen ratios in rumen fluid and bacteria suggested that, for all the diets except diet B, 55–80 and 40–50% of the non-ammonia nitrogen in rumen fluid was of microbial origin for the calves and cows respectively.

Nucleic acid metabolism in the ruminant. Determination of nucleic acids in digesta

1. Procedures, based on those of Schmidt & Thannhauser (1945) and Schneider (1945), for the extraction and estimation of nucleic acids in bovine digesta were examined in detail.

2. Final methods which were suitable for routine determination of RNA and DNA were essentially as follows. Digesta samples were extracted in the cold, first with a solution of trichloroacetic acid in ethanol, then with aqueous trichloroacetic acid solution and finally with lipid solvents. The dried residue was hydrolysed with alkali, purified by passage through a Dowex resin, and the RNA, in the form of mononucleotides, determined by U.V. absorption. DNA was determined separately in hot perchloric acid extracts of the original dried residue by colorimetric estimation of the deoxyribose content.