The Nutritive Value of Pasture Proteins

VI. Digestibility of Proteins in Orchard Grass at Various Growth Stages-Investigation Using Two Different Pepsin-Digestion Methods

Abstract

The proteins of orchard grass (Dactylis glomerata) at various growth stages were examined for digestibility by the two different pepsin-digestion methods described below. Then the cause of the difference in digestibility between the two methods was discussed.
The methods were carried out as follows.
"Digestion A": A grass sample was ground by a blendor and extracted successively with water, 0.3% NaOH, and hot-0.3%-alkaline-60%-ethanol. Protein nitrogen "a" was determined in each extracted fraction (and the insoluble fraction). On the other hand, the grass ground wasd igested by pepsin at 37°C for 48 hours in citrate-HCI buffer (pH 1.2), extracted by solvents, and determined for protein nitrogen "b" in each fraction. Then the digestibility of each Protein was calculated from the formula a-b/a×100.
"Digestion B": An aliquot of the extracted fraction was used to determine the protein nitrogen content "c" of the fraction. Another aliquot was incubated with pepsin at 37°C for 48 hours, the incubation mixture having been adjusted to pH 1. 2, and used to determine residual protein nitrogen "d". Digestibility was then calculated from the formula c-d/c×100.
Results obtained are as follows.
1. The digestibility of water-soluble protein was generally much lower in "digestion B" than in "digestion A". In NaOH-soluble protein, digestibility was generally much higher in "digestion B" than in "digestion A". In hot-alkaline-ethanol-soluble protein, digestibility was higher in "digestion A" than in "digestion B". In insoluble protein, digestibility was much lower in "digestion B" than in "digestion A".
2. The sequence of digestibility in "digestion B" among proteins in the grass was as follows: water-soluble>NaOH-soluble>hot-alkaline-ethanol-soluble>insoluble. The sequence of digestibility in "digestion A" was as follows: water-soluble>hot-alkaline-ethanol-soluble>insolubl>NaOH-soluble.
3. The above-mentioned results seem to, suggest that not only digestion of proteins but also some change in solubility of the proteins took place during the process of "digestion A". That is, it seems that water-soluble and hot-alkaline-ethanol-soluble proteins lost their solubility in each solvent, and that a part of the NaOH-soluble protein in the digested sample was derived from some other protein(s).
4. The digestibility of the whole protein (the sum of the extracted proteins and insoluble protein) calculated in "digestion B" was considerably lower than that calculated in "digestion A:. This result suggests that there was a change in property of the proteins during the extraction.
5. Digestibility in "digestion B" decreased in NaOH-soluble and hot- alkaline-ethanol-soluble protein and whole protein as grass grew in the first growth stage. This shows a possibility that a change occurred in the quality of these proteins during the growth of grass.