Oh Oh

Figure 4. Structure of 5'-ribonucleotides investigated for synergistic biochemical effect with l-glutamate.

Figure 5. The synergistic effect shown as the relative intensity of umami taste to MSG, changing the ratio of 5'-IMP to MSG while the total concentration of MSG + 5'-IMP remained constant as 0.05 g/dL.

This intensity of umami of L-glutamate when IMP coexists is expressed by the following relation (5):

y = n + 1,200p v where y is the concentration of L-glutamate (%) that shows the same intensity of umami as that shown by the mixture of L-glutamate and IMP; ¡u is the concentration of L-glutamate in mixture (%); and v is the concentration of IMP in mixture (%), respectively.

When using 5'-guanylic acid (GMP) in place of IMP in the mixture, the relation of intensity of umami was summarized as follows:

where v' is the concentration of GMP in mixture (%).

The concentration—response relationship for L-glutamate does not follow the simple taste equation obtained under the assumption that there exists only one type of receptor site for L-glutamate. Assuming that two types of receptor sites exist—with different dissociation constants for L-glutamate—in rat taste cell membrane we used the following equation (12):

Here, R is the magnitude of the response; i?H and RL are the magnitudes of the maximal responses brought about by occupation of high- and low-affinity sites, respectively; and S is the concentration. The best-fitted curves for the data were obtained with a computer utilizing a program for the linear square iterative fitting of the preceding equation. The parameters for the curves were determined as follows:

l-glutamate alone l-glutamate + 0.1 mM GMP

The values of i?H = 1.18, and Rh — 15.59 were unchanged by the presence of 0.1 mM GMP.

There seem to be specific binding sites for purine-based 5'-nucleotides in the taste receptor membranes, because only the specific nucleotides have the ability to show the synergism. The binding sites for the nucleotides seem to be part of or closely associated with L-glutamate receptors.

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