Walnut kernels are health-promoting nuts, which are mainly attributed to polyunsaturated fatty acids, phenolics, and phytosterols. However, the information concerning benefits of walnut proteins are limited. In this study, endopeptidases, aminopeptidases, carboxypeptidases, superoxide dismutases, catalases, and phospholipases with respective relative abundance of 2.730, 1.728, 0.477, 3.148, 0.743, and 0.173‰ were identified by liquid chromatography tandem mass spectrometry. These endogenous proteases exhibited activity in a broad pH range of 2-6.5, and optimal at pH 4.5 and 50 °C. Aspartic endopeptidases were predominant endopeptidases, followed by cysteine ones. There were two types of aspartic endopeptidases, one (not inhibited by pepstatin A) exerted activity at pH 2-3 and the other (inhibited by pepstatin A) optimal at pH 4.5. Carboxypeptidases were optimal at pH 4.5, and aminopeptidases exerted activity at pH near 6.5. These endogenous proteases assisted the digestion of walnut proteins, and soaking, especially peeling, greatly improved the in vitro digestibility.
Keywords: Aminopeptidases; Aspartic endopeptidases; Carboxypeptidases; Cysteine endopeptidases; Raw walnut kernels; Superoxide dismutases.
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