Proteomic Quantitative UV Absorption Spectrum Analysis of Effect of Heat Stress on Protein Extract from Cowpea Seed (Vigna unguiculata (L) Walp)

Abstract

Aim: Proteomic quantitative UV absorption spectrum analysis was used to study the effect of heat stress on protein extract from cowpea seeds (Vigna unguiculata (L) Walp). Study Design: Protein extracts were obtained from 9 cowpea accessions obtained from GeneBank of International Institute of Tropical Agriculture in January 2014. Each protein extract was divided into four batches out of which three batches were subjected to different temperature treatments and incubation at 37°C, 60°C and 100°C for 1 hour and the remaining one batch served as control. Protein content in each control protein extract and 37°C, 60°C and 100°C treated protein extracts from each of the 9 cowpea samples were determined at 280 nm using bovine serum albumin standard curve. Place and Duration of Study: Biochemistry Unit, Department of Chemical Sciences, Afe Babalola University Ado Ekiti, Nigeria between January 2014 and June 2014. Methodology: A200-A960 UV wavelengths absorption spectrum analysis was carried out on control protein extract and 37°C, 60°C and 100°C treated protein extracts respectively from each of the 9 cowpea samples. In order to establish the relationship between protein extracts (control) and protein extracts heat treated (37°C, 60°C, and 100°C), cluster analysis of optical density (OD) data was carried out using numerical taxonomy and multivariate analysis system. Results: The protein content (control) in Vigna unguiculata (L) Walp was between 8.4 and 10.8 mg/ml (10.5-13.5%) in seed, while protein content (heat treated) in Vigna unguiculata (L) Walp was between 8.9 and 9.5 mg/ml (11.2-11.9%), 8.7-9.5 mg/ml (10.9-11.9%), 9.0 and 11.8 mg/ml (11.3-14.7 %) in  heat treatments of 37°C, 60°C, and 100°C respectively. The protein UV absorption spectra of control protein extract and 37°C, 60°C and 100°C treated protein extracts from each cowpea accession were generally different due to differential UV wavelength protein absorption. Cluster analysis of absorbance spectra optical density values revealed five clusters (cluster 1, cluster 2, cluster 3, cluster 4, and cluster 5) among control protein extracts and protein extracts heated at 37°C, 60°C, and 100°C. Cluster1 was made up of protein extracts heated at 37°C and 60°C, while cluster2 and cluster3 constituted closely related protein extracts heated at 37°C and 100°C respectively. Cluster4 was typical of control protein extracts, while cluster5 was made up of distinct protein extracts heated at 100°C. Conclusion: Heating protein extracts at 37°C, 60°C, and 100ºC has altered proteomic diversity in different cowpea accessions and this could make protein extraction more difficult with implications on protein properties.