Measurement of Nitrate‐ and Nitrite‐reductase Activities in Whole Cells of Chlorella

Summary A method is described for the assay of nitrate‐reductase activity in frozen and thawed cells of Chlorella. The method depends on coupling nitrate reductase with malic dehydrogenase, both enzymes and the necessary coenzyme being retained within frozen/thawed cells. Other ways than destroying permeability by freezing are considered and a short treatment of fresh cells with 1.5 M glycerol is shown to be effective. The method has been used to follow the induction of nitrate‐reductase activity in Chlorella. Other enzyme activities that can be measured by similar methods, i.e. by enzymic coupling within damaged cells, are nitrite reductase and urea amidolyase.