Characterization of sea cucumber (stichopus japonicus) ovum hydrolysates: calcium chelation, solubility and absorption into intestinal epithelial cells

BACKGROUND: Sea cucumber (Stichopus japonicus) ovum hydrolysates (SCOHs) chelated with calcium were produced to investigate the characteristics of calcium binding and solubility, as well as to study any effects on calcium absorption by human intestinal epithelial cells. RESULTS: The results of the present study show that the calcium-binding capacity of SCOHs depended greatly on the type of proteases. The maximum level of Ca binding (0.38 mmol L^–1) occurred when trypsin was used, with a peptide yield of 85.7%. Investigation of the possible chelating modes between SCOHs and calcium ions indicated that calcium ions bound to SCOHs primarily via interactions with carboxyl oxygen and amino nitrogen atoms of Glu and Asp and also that the phosphoserine residues might be also responsible for SCOH–calcium chelation. Moreover, SCOH–calcium complexes maintained the solubility of calcium under simulated gastrointestinal digestion, regardless of the presence of dietary components such as oxalate. Furthermore, SCOH–Ca led to higher peak intracellular [Ca²⁺]_i in both Caco-2 cells (338.3 nmol L^–1 versus 269.6 nmol L^–1) and HT-29 cells (373.9 nmol L^–1 versus 271.7 nmol L^–1) than casein phosphopeptide-Ca. CONCLUSION: Carboxyl oxygen and amino nitrogen atoms in the SCOHs could bind calcium ions, forming SCOH–calcium complexes. These complexes improved calcium solubility under simulated gastrointestinal digestion and also promoted calcium absorption in Caco-2 and HT-29 cells.