Determination of the nutritional value of proteins obtained from Ulva armoricana

Abstract

The in vitro digestibility of Ulva armoricana proteins by trypsin, chymotrypsin and human intestinal juice was determined to evaluate their nutritional value. The amino acid composition of the protein fraction and its changes during a sampling period from October to February were also studied. Some differences in the protein pattern shown by SDS PAGE were found in different months, such as the presence of a 54 kDa protein in February. The protein fraction is composed mainly of aspartic and glutamic acids (24–35% of protein fraction, according to season) and the essential amino acids constitute 27–36% of the total fraction. The efficiencies of trypsin and chymotrypsin in Ulva protein digestion are comparable. Only four proteins with apparent molecular weights of 86, 68, 40, and 29 KDa are digested by these proteolytic systems. The proteins from the October sample were more sensitive to chymotrypsin than those from the February sample. For instance, two proteins with apparent molecular weights of 100 and 67 kDa were weakly digested by chymotrypsin in the February extract, were fully digested in the October sample. The February sample differed from two others in the presence of glycosylated proteins, most of which have apparent molecular weights higher than 43 KDa. With the October sample, the activity of human intestinal juice was more effective than two other proteolytic systems. This is especially evident with a 27 kDa protein, which was only partially digested by the intestinal liquid and not digested by chymotrypsin or trypsin. However, human intestinal juice in the February apparently did not attack the 27 kDa protein. These data suggest a change in protein structure making it less sensitive to human intestinal juice. The glycosylation of protein extract, which was especially marked in February, could explain the differences in behaviour of U. armoricana proteins in response to the digestive action of human enzymes.