Influence of the Maillard-type caseinate glycation with lactose on the intestinal barrier activity of the caseinate digest in IEC-6 cells
Protein glycation via the Maillard reaction has been used excessively in recent years to modify protein properties. A milk protein caseinate can be glycated with lactose through this Maillard reaction. In this study, two tryptic digests from the lactose-glycated caseinate and commercial caseinate were assessed and compared for their activities in the intestinal barrier function of a non-transformed small intestine epithelial cell line (IEC-6 cells) as a cell model, to assess if this glycation might have positive or negative impact on caseinate biofunctions. Cell treatment with the two digests led to promoted cell growth by 103.1–130.9 (24 h) or 106.4–133.2% (48 h). More importantly, the two digests improved intestinal epithelial barrier integrity, reflected by the increased trans-epithelial electrical resistance values of 109.0–123.6 (24 h) or 112.5–127.8% (48 h), decreased epithelial permeability by 75.1–94.4 (24 h) or 64.0–83.4% (48 h), reduced bacterial translocation in the cells, and exhibited higher antibacterial activity against Escherichia coli. Furthermore, the results from the conducted immuno-fluorescence, qPCR, and western-blot assays indicated that the two digests enhanced the expression levels of 6 tight junction proteins ZO-1, ZO-2, occludin, claudin-1, claudin-3, and claudin-4 in the cells. However, the data comparison results also showed that the lactose-glycated caseinate digest always showed lower efficiency than the caseinate digest to improve the intestinal epithelial barrier function of this cell model. Thus, the present results highlight that the Maillard-type lactose glycation of caseinate gives the resultant tryptic digest with impaired activities in the intestinal epithelial barrier function of this cell model, suggesting that another adverse effect of the Maillard reaction on food proteins should be reconsidered and revised.