Antioxidant and type II diabetes related enzyme inhibition properties of methanolic extract of an underutilized food legume, Canavalia ensiformis (L.) DC: Effect of traditional processing methods

Abstract

The methanolic extract of Canavalia ensiformis (L.) DC (Jack bean) seed, an underutilized food legume collected from India was analyzed for antioxidant and health relevant functional properties. The raw seeds contained total free phenolic content of 12.98 ± 1.63 g catechin equivalent/100 g extract DM. The raw seed extract exhibited ferric reducing/antioxidant power (FRAP, 1218 mmol Fe[II]/mg extract), inhibition of β-carotene degradation (49.81%), radical scavenging activity against DPPH (56.78%) and superoxide (35.89%). In addition, 77.56% of α-amylase and 75.45% of α-glucosidase enzyme inhibition characteristics were found under in vitro starch digestion bioassay. Sprouting + oil-frying caused an apparent increase on the total free phenolic content with significant improvement on antioxidant and free-radical scavenging capacity, while soaking + cooking as well as open-pan roasting treatments showed diminishing effects. Inhibition of α-amylase and α-glucosidase enzyme activities were declined to 22.69 and 42.69%, respectively during sprouting + oil-frying treatment is more desirable for the dietary management of type II diabetic patients.

Introduction

Natural antioxidants from plant-based sources offer an alternative source of dietary ingredients to promote healthy life. For example, α-amylase and α-glucosidase inhibitors are known to regulate/prevent hyperglycemia linked type II diabetes. In this research health relevant functionality of seeds from Canavalia ensiformis (L.) DC (commonly known as jack bean) is explored. The C. ensiformis originated from South America and grown in the tropics and subtropics produces high yields especially in regions of low altitude, high temperature and relative humidity (Molina, Argueta, & Bressani, 1974). Although some may be cultivated in drought-prone areas of Arizona and Mexico, C. ensiformis is considered one of the few pulses that grow well on highly leached, nutrient-depleted low land tropical soils. C. ensiformis is often grown on walls as an ornamental plant in the North Coast of Columbia and Nigeria (Udedibie, 1990) and as cover crop in western countries. Total yield of forage and dry seeds of C. ensiformis range from 1 to 10 t/ha/yr (Kessler, 1990).

The plant is often used as high protein food and forage crop in Southwestern United States, Mexico, Central American countries, Brazil, Peru, Ecuador and West Indies (Sauer & Kaplan, 1969). C. ensiformis seed comprises of 84–89% cotyledon and 11–15% of seed coat (Sridhar & Seena, 2006) and seed coat colour range from maroon to red (Janardhanan, Vadivel, & Pugalenthi, 2003). Indian tribal sects, Kurumba, Malayali, Irula and other Dravidian groups cook the mature seed as food (Janardhanan et al., 2003), the westerns consume immature pods of C. ensiformis as vegetables, and the seeds are roasted and grounded to make coffee-like drink (Bressani, Brenes, Gracia, & Elias, 1987). The proximate composition of the jack bean seeds are 24–32% crude protein, 1.8–9.6% crude lipid, 4.65–10% crude fibre, 2–4.6% ash and 43–60% carbohydrates (Janardhanan et al., 2003). It is known to have anti-nutritional compounds such as concanavalin A, l-canavanine, canatoxin, polyamines, protease inhibitors, flatulence factors, cyanides, saponins, urease and l-Dopa.

The seed decoction or powdered seeds from C. ensiformis are used as an antibiotic and antiseptic (Gill & Nyawuame, 1994). Proteins with complete sequence homology to bovine insulin present in C. ensiformis seed coat are used as antihuman insulin antibodies (Oliveira, Sales, Machado, Fernandes, & Xavier, 1999). It can be used as treatment for diabetes as it has been shown to significantly lower the blood glucose level in alloxanized mice. Seed proteins of C. ensiformis have considerably decrease cholesterol level in hypercholesterolemia rats (Marfo, Wallace, Timpo, & Simpson, 1990) and Marchetti, Mastromarino, Rieti, Seganti, and Orsi (1995) found inhibition of herpes simplex, rabies and rubella viruses by C. ensiformis lectins. In addition, Swaffar and Ang (1999) have also demonstrated the inhibitory effect of l-canavanine isolated from C. ensiformis against MIA PaCa-2 pancreatic cancer cells.

There can be no doubt that nutritional value of seeds and medicinal properties of different parts of C. ensiformis plant are greatly reported, but there has been very little information on antioxidant and medicinal properties of seed materials. Hence, the present research attempts to analyze the total free phenolics content, antioxidant and type II diabetes related enzyme inhibition properties of methanolic extract of raw and traditionally processed C. ensiformis seeds.