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Dopamine
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Griffiths (1959) identified dopamine (3,4-dihydroxyphenylethylamine) as the substrate for the polyphenol oxidase of banana. DOPA (3,4-dihydroxyphenylalanine) was not detected. Later (1961) Griffiths found that the concentration of dopamine in bananas is determined by the extent of the contribution of the acuminata genome to the genotype. Waalkes et al. (1958) measured the content of dopamine in bananas and found 700 ug/g of peel and 7.0 ug/g of pulp.
Barnell and Barnell (1945) located what they referred to as tannins, although not specifically dopamine, in the latex vessels of the skin and pulp and in scattered isolated parenchyma cells in three-fourths full, green bananas. After the fingers had sprung, i.e., the yellow began to show through the green of the skin, the latex in the pulp tended to dry out and the tannins gradually disappeared from the dried latex contents. Little change was observed between the eating-ripe and overripe stages. Buckley (1964) reported the dopamine content in peel of developing bananas. Ten to 15 percent accumulated prior to emergence (0.7 mg/g fresh weight) and 85 to 90 percent (2.0 to 3.5 mg/g fresh weight) accumulated during the first month after emergence. Half of the dopamine was located in the outer millimeter of peel. Dopamine decreased 20 percent with ripening. However, the dopamine content of the inner 2 to 3 ml of peel increased 35 percent with ripening so that the content per entire peel seemed to remain constant to harvest. The peel enlarged during this period, giving a final concentration of 1.0 to 1.2 mg/g fresh weight. Dopamine concentration was least in the end attached to the stem and greatest in the floral end of the fruit.
Dopamine is synthesized from tyrosine. In mammalian systems dopamine is formed by the decarboxylation of DOPA formed from tyrosine through the action of tyrosine hydroxylase. However, in bananas tyramine is the intermediate compound (Buckley, 1964). Deacon and Marsh (1971) isolated an enzyme from bananas that appeared to be the hydroxylase that converts tyramine to dopamine.
Waalkes et al. (1958) and Udenfriend et al. (1959) found a high content of other physiologically active amines in bananas. These do not act as substrates for polyphenol oxidase, however.
Updated: Wednesday, June 20, 2007.
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