Cmp

124 ± 16

Sodium phosphate

108 ± 9

Note: Concentration, 10 mM; and pH 6.0.

Note: Concentration, 10 mM; and pH 6.0.

is ineffective (11). The pyrimidine nucleotides are reportedly not effective (3). With the bovine system, the order of effectiveness was GMP > IMP > UMP, and XMP, CMP, and AMP were effective (Fig. 2). Additional structure-activity considerations were studied by examining the effects of the free bases, the nucleoside, and the di- and triphosphates of GMP and AMP. The data in Figure 3 show that only 5'-GMP was effective in increasing binding of L-

glutamate. The regions of the molecule that appear to be critical in the interaction with the regulatory site are shown in Figure 4. The keto function on the aromatic ring appears critical. With an amino group at this position, there is no synergistic activity. The data from human studies show that 5'-GMP and 5'-IMP are potent synergisti-cally, 5'-XMP is less effective, but 5'-UMP is not effective. The binding studies agree with the earlier human psychophysical results showing the importance of the 4- or 6-po-sition on the ring, the 2-position on the ring, and the 5'-monophosphate group on the ribose. There are two points of difference in the bovine and human data: (1) XMP did not enhance binding but is synergistic in humans, and (2) UMP increased binding but is not effective in humans.

The characteristics of the chemical structure of nucleotides attributed to the taste of umami for humans are summarized as follows: (1) a purine base on which the OH-bond is located at the 6-position carbon, and (2) a pentose on which the phosphate bond locates at 5'-position of the OH-bond.

When the total concentration of L-glutamate and IMP is kept constant (0.015 g/dL), the intensity of umami increases with the increasing ratio of IMP and goes to maximum when the ratio of IMP is 50%; it then decreases with the increasing ratio of IMP, which is shown in Figure 5.

n

n

r—i

1-■

n

a)

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