The following comes directly from Wikipedia:
Disodium guanylate (E627), chemical formula C10H14N5O8P, is a food additive used as a flavor enhancer, in synergy with monosodium glutamate (the sodium salt of glutamic acid, MSG). As it is a fairly expensive additive, it is not used independently of glutamic acid; if disodium guanlyate is present in a list of ingredients but MSG does not appear to be, it is likely that glutamic acid is provided as part of another ingredient such as a processed soy protein complex.
Disodium guanylate is a flavor enhancer derived from dried fish or dried seaweed. It is a by-product of disodium inosinate.
SDSU Professor Ronald A. Newcomb looked into this for us and said; " U.S. Patent 4958017 gives a method for synthesizing this substance and says in part, 'An industrially advantageous process for producing an inosine-guanosine mixture having a higher weight ratio of guanosine/inosine than a starting mixture of nucleoside crystals and inosine substantially free of guanosine from which comprises adjusting an aqueous fluid containing 10 to 30% by weight/volume of a nucleoside mixture to a pH of 9.1 to 9.5, wherein the total amount of inosine and guanosine is more than 95% by weight based on dry matters and the weight ratio of guanosine/inosine is 0.5 to 1, separating the resultant solids and then crystallizing inosine from the resulting solution.' Now, this is not at all alarming since these are nucleic acids which are also flavor enhancers. Like MSG, too much becomes a problem and they may trigger reactions in certain populations, but they are useful to the body, much like an isolated amino acid you would find in the supplement section of your health food store. However, the inclusion of this item, usually Disodium inosinate and Disodium guanylate together indicate there is a source of MSG in the product which is likely, and deceptively, not listed."