Phenylc3 Compounds

One of the first important derivatives of phenylpyruvic acid is the 'essential' amino-acid phenylalanine, produced with the aid of pyridoxamine (Figure 2.17, with that reaction running in reverse). It is an essential amino-acid because all animals must have it in their diet. Tyrosine in plants is made directly from prephenic acid by oxidation concurrently with decarboxylation. Mammals can make tyrosine from phenylalanine, but insects must obtain it in food. Some other simple phenyl-C3 compounds and derivatives are shown in Figure 8.2.

The mechanism of formation of tyrosine in animals is shown in Figure 8.3. An epoxide of phenylalanine is formed which is opened in two ways. Labelling experiments show that it goes about 90% with a 1,2-shift of hydrogen from para to meta position. This pathway is known as the NIH shift, from the National Institutes of Health (USA) where it was discovered. The remaining 10% gives direct loss of hydrogen from the para position. There is evidence from a number of experiments that insects cannot convert phenylalanine to tyrosine, but can hydroxylate tyrosine further.

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