CHEMICAL AND BIOCHEMICAL STUDIES ON LEAF PROTEINS

Abstract

Six leafy green vegetables commonly incorporated into diets by the Ibos of Anambra and Imo States of Nigeria have been used in this study. The vegetables were: Telfairia ocoidentalis, Celosia ,argeltea 'green' Celosia argentea 'red, Solanum incanum, Solanum anomalum, and Curcubita pepo. Leaf protein concentrates, chloroplastic and cytoplasmic protein fractions have been extracted from these leafy vegetables. Investigations covering the composition and nutritional qualities of the leaf protein concentrate and the extracted protein fractions were carried out. The protein content of the vegetables ranged from 17% in Solanum incanum to 27% in Telfairia occidentalis and Celiosia argentea 'red'. The leaf protein concentrates had a crude protein content ranging from 45% to 57%. The high protein content of the leaf protein concentrate, coupled with the high protein/fibre ratio suggest that these protein concentrates can be potential sources of dietary protein. The leaf protein concentrates had lipid content ranging from 26% to 29%; carbohydrate content ranging from 9% to 21% and ash content of 1% to 29%. The protein content of the cytoplasmic protein fraction) leaf protein concentrates) chloroplastic protein fractions) vegetables. The investigation covered the analyses of the minerals (Na, K, Ca, Mg, Mn. Fe, Cu, Zn, P). Atomic absorption spectro-photometric analyses reveal the presence of a wide spectrum of mineral elements in the vegetables, leaf protein concentrates and Chloroplastic protein fractions. There is a wide variation in the distribution of these mineral elements in each and between the various fractions, however, those fractions are good sources of Ca, Fe and P. The cytoplasmic protein fraction was virtually devoid of mineral elements except for the consistent presence of P in all the fractions. The nutritive values of the leaf protein concentrates were evaluated by the rat assay method. The procedure involved the measurement of the Biological value (BV), True digestibility (TD), Protein efficiency ratio (PFM) and the Net Protein Utilization (NFU). Thu leaf protein concentrates were found to have lower nutritive values than the standard soya protein diet of Bassir and Leobel (1968). Phenolic compounds occur in the leaf protein concentrates and these may lower the nutritive values of the concentrates. The leaf proteins contain a variety of individual proteins having a molecular weight range of 23,000 to 100,000. There is, however, in each leaf a protein component with a molecular weight of over 100,000. Fractionation of the leaf proteins by Sephadex gel chromatography yielded four fractions with absorbance characteristic of proteins. The presence of proteins were detected in the first three fractions whilst the whilst the fourth fraction was almost devoid of proteins, Polyacrylamide gel electrophoresis has been used to study the protein composition of the leaf proteins. Separation of the various leaf juices on 5% acrylamide gels gave 8 to 11 protein bands. The nature of the buffer used in extracting the proteins, the fixative and staining procedures used, the storage time of the gels and the composition of the gels influence the resolution attained. The amino acid composition of the various leafy vegetables were similar and resembles those of their respective leaf protein concentrates Methionine was, however, the limiting amino acid in all the cases. Variations in amino acid content do exit and these variations were greater for some amino acids than others. The method of precipitation of the bulk loaf proteins influences its dispersibility in water. The nitrogen solubility studied on the leaf protein concentrates reveals that those concentrates have a minimal solubility in the pH range 4.2 to 5.0. They however have a high nitrogen solubility in the acidic and alkaline pH values. This nitrogen solubility is, however, influenced by the concentration of the leaf protein concentrates. The results of these studies are discussed.