Eating satiating,
protein-rich foods is one of the key aspects
of modern diet, although a bitter off-taste often limits the application
of some proteins and protein hydrolysates, especially in processed
foods. Previous studies of our group demonstrated that bitter-tasting
food constituents, such as caffeine, stimulate mechanisms of gastric
acid secretion as a signal of gastric satiation and a key process
of gastric protein digestion via activation of bitter
taste receptors (TAS2Rs). Here, we tried to elucidate whether dietary
non-bitter-tasting casein is intra-gastrically degraded into bitter
peptides that stimulate mechanisms of gastric acid secretion in physiologically
achievable concentrations. An in vitro model of gastric
digestion was verified by casein-fed pigs, and the peptides resulting
from gastric digestion were identified by liquid chromatography–time-of-flight-mass
spectrometry. The bitterness of five selected casein-derived peptides
was validated by sensory analyses and by an in vitro screening approach based on human gastric parietal cells (HGT-1).
For three of these peptides (YFYPEL, VAPFPEVF, and YQEPVLGPVRGPFPIIV),
an upregulation of gene expression of TAS2R16 and TAS2R38 was observed. The functional involvement of these
TAS2Rs was verified by siRNA knock-down (kd) experiments in HGT-1
cells. This resulted in a reduction of the mean proton secretion promoted
by the peptides by up to 86.3 ± 9.9% for TAS2R16kd (p < 0.0001) cells and by up to 62.8 ± 7.0% for TAS2R38kd (p < 0.0001) cells compared with mock-transfected cells.
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