Interpretation of the rate of density labelling of enzymes with 2H2O. Possible implications for the mode of action of phytochrome.

The interpretation of the rate of density labelling of enzymes (the differential or comparative density labelling technique) is considered in terms of two simple models of enzyme turnover. On the basis of this illustrative analysis it is demonstrated that the interpretation of differential (comparative) density labelling experiments is critically dependent on the establishment of a definitive mechanism of enzyme turnover for each particular enzyme studied. In the light of these findings the interpretation of comparative (differential) density labelling experiments appertaining to the mechanism of phytochrome-mediated increases in enzyme activity in higher plants is re-assessed.