“Antioxidant activity and characterization of protein fractions and hydrolysates from normal and quality protein maize kernels”

Highlights

• Antioxidant activity of proteins and hydrolysates differ between varieties of maize.

• Proteins and hydrolysates from QPM present higher antioxidant potential.

• Improved varieties of maize are potential sources of antioxidants.

Abstract

Maize is the main crop cultivated worldwide with more than 1 billion metric tons produced annually and is one of the most relevant sources of protein for human consumption in developing countries. Proteins and peptides isolated from maize exert relevant antioxidant activity which is increased by enzymatic hydrolysis. However, there is limited information about the antioxidant potential of proteins isolated from Quality Protein Maize (QPM) varieties and their hydrolysates. The aim of this research was to determine the differences in protein profile and antioxidant activity of protein fractions and hydrolysates between a hybrid white maize (Asgrow 773) and a QPM variety (CML-502). The biophysical evaluation and the total protein quantification by Kjeldahl and fractions by ninhydrin were consistent with the changes due to the breeding process of the QPM material. The antioxidant potential of the hydrolysates obtained from albumins and globulins had a 3-fold increase in both maize varieties. The prolamins hydrolysates presented an increase of 7-fold for the normal variety and 2-fold for the QPM variety. The results of this research allow indicate that the QPM varieties are a source of antioxidant peptides and promising candidates in the search for proteins and peptides with other bioactivities.

Introduction

Maize is the main crop cultivated worldwide with a production exceeding 1021 million tons in 2014 (http://faostat3.fao.org, accessed January 2016). Protein quality and content in maize is poor compared to legumes, mainly due to the lower levels of lysine and tryptophan detected in this cereal. However, maize is a staple crop and one of the most relevant sources of protein for human consumption in developing countries, where other protein-rich sources are either not available or not affordable (Ignjatovic-Micic et al., 2015). Cereal proteins are highly complex and heterogeneous; they establish a broad range of inter-protein interactions. The normal varieties of maize (non protein-enriched) contain an average of 10% protein mainly located in the endosperm and germ (Nuss and Tanumihardjo, 2010). The maize proteins are classified into four fractions based on their solubility: prolamins, albumins, globulins, and glutelins. The prolamins or zeins which are stored in protein bodies are the main seed protein, and exhibit solubility in ethanol and insolubility in water (Landry and Moureaux, 1980, Malumba et al., 2008). The albumins and globulins are mainly localized in the germ and are considered biologically active proteins because they regulate seed metabolism. Albumins are hydrosoluble, whereas globulins are soluble in diluted salt solutions and glutelins are soluble in alkaline buffers. The amino acid profiles of the four protein fractions from maize are unique, therefore it is expected that their hydrolysates or peptides will contain different amino acid sequences and properties (Malumba et al., 2008, Nuss and Tanumihardjo, 2010).

Several studies have shown that food proteins are an important source of bioactive peptides which are inactive in the native protein conformation. However, once subjected to enzymatic hydrolysis, the released peptides may exert different biological activities (Ortiz-Martinez et al., 2014). Particularly, hydrolysis with alcalase has proven to be an effective approach for the generation of antioxidant peptides from plant proteins (Liu et al., 2015).

The QPM varieties present a reduction in the concentration of prolamin fraction associated with an increase in the soluble fractions content. These changes are linked with a better protein quality, which is enriched with higher concentrations of the limiting amino acids lysine and tryptophan. Improved protein quality has been correlated with the presence of the mutant gene opaque-2 (Serna-Saldivar et al., 2008). There are several investigations aimed towards the isolation of peptides from corn gluten meal and zein with antioxidant and other relevant bioactivities such as antiproliferative, antimicrobial or ACE (Angiotensin-converting enzyme) inhibition (Liu et al., 2015). Despite the extensive study of the antioxidant potential of secondary metabolites from maize and proteins isolated from its industrial by-products, little has been studied about the possible antioxidant properties of proteins and hydrolysates obtained from whole kernels of QPM varieties. The hypothesis of this work was that known differences in the protein composition of QPM compared to normal hybrids results in a significant increase in the antioxidant potential of both its proteins and hydrolysates. Therefore, the aims of this research were: to determine and compare the protein profiles of a QPM and a normal hybrid maize and to evaluate the possible effects on the antioxidant activity of the different protein fractions and their hydrolysates.