Influence of vitamin B12 on the nutritive value of vegetable proteins

Abstract

The work reported in this thesis relates to two main aspects: (a) To confirm and extend the finding that vitamin B?? counteracts the adverse effects of the tryptic and growth inhibitors present in raw soyabean. (b) To examine the role of vitamin B?? in protein metabolism and to determine the conditions, if any, under which vitamin B?? can exert a favourable effect on nitrogen utilisation.

An exhaustive survey of the literature was carried out before initiating the investigations. A brief account of the work done by others is given under appropriate headings.

The materials used and the methods employed in these studies are described in detail.

The biological value of raw soyabean protein, with and without vitamin B??, was determined by the usual nitrogen?balance technique. It was found that the biological value of raw soyabean protein supplemented with vitamin B?? improved to the same extent as the biological value of optimally autoclaved soyabean, in which the tryptic inhibitor is completely destroyed. This suggests that vitamin B?? inactivated the inhibitor that interfered with the utilisation of the protein.

It was also found that the regeneration of haemoglobin by raw soyabean protein with vitamin B?? was of the same order as the regeneration obtained with autoclaved soyabean protein. These data not only confirmed the earlier observation but also extended it.

At the same time, it was observed that no extra benefit, in terms of increased haemoglobin regeneration, could be obtained by adding vitamin B?? to a diet composed of autoclaved soyabean or casein.

In vitro studies on the rate of release of amino acids, and methionine in particular, showed that amino?acid liberation from raw soyabean was generally enhanced by the addition of vitamin B??. Such an effect would be expected to enhance the nutritive value of the protein.

The data on methionine release showed that, under the influence of vitamin B??, this amino acid was made available at a rate that facilitated mutual supplementation with other amino acids. The usefulness of the protein would therefore be enhanced.

In an attempt to explain the mechanism by which vitamin B?? improves the nutritive value of raw soyabean protein, studies were carried out to determine the amino?acid content of the intestines of rats at different intervals after feeding the raw?soyabean diet with and without vitamin B??. It was found that, uniformly, the concentration of amino acids was much lower in the vitamin?B???supplemented rats. From these observations, it was concluded that vitamin B?? increased the rate of digestion in vivo of raw soyabean protein in the early stages of the process. It was postulated that vitamin B?? inactivated the inhibitor in raw soyabean by forming a complex with it, thus removing it from the sphere of action.

It was naturally of interest to determine whether vitamin B?? could counteract tryptic inhibitors present in other legumes. Experiments carried out with cowpea showed that vitamin B?? was not able to overcome the effect of the inhibitor contained in cowpea, which was found to have a much higher potency than the inhibitor present in raw soyabean.

Raw pea was found to possess poor antitryptic activity. Although growth studies showed that the nutritive value of raw pea protein improved when supplemented with vitamin B??, this could not be confirmed by nitrogen?balance studies. Thus, the action of vitamin B?? in counteracting the growth and tryptic inhibitors appeared to be specific to soyabean only.

It was felt that a re?examination of the possible role of vitamin B?? in protein metabolism would yield useful information, especially in view of the controversy regarding its exact role in protein and fat metabolism. It was found that vitamin B?? did not improve the biological value of proteins such as groundnut, sesame, and pea. However, the biological value of autoclaved soyabean protein was definitely improved. Again, this indicated that the action of vitamin B?? on soyabean protein is specific in nature.

Further studies showed that the level of choline in the diet influenced the biological value of autoclaved soyabean protein. It was found that the requirement of the rat for choline chloride was not more than 200 mg per 100 g of diet.

In summary, while it may not be justifiable to draw general conclusions on the role of vitamin B?? in protein metabolism, the data collected clearly show that:

Vitamin B?? is able to counteract the adverse effects of the growth and tryptic inhibitors present in raw soyabean. Vitamin B?? improves the utilisation of soyabean protein.