Activation of the zymogen of collagenase by Ehrlich-ascites-tumour-cell-surface trypsin-like enzyme.

dehydrogenase (Rose et al., 1969). The ,H labelling of ['HI lactate in 2H,0 and the 'H labelling of [2H]lactate in 'H,O has been studied. A comparison of the exchange in whole cells and lysates has also been undertaken. The detailed measurement of exchange kinetics permitted by the n.m.r. technique has revealed the dependence of the exchange on the different enzyme activities and on the triose phosphate concentrations. The isotope-exchange reaction involving the C-2 position of lactate can be inhibited by iodoacetate and acetamide. The locus of this inhibition is glyceraldehyde phosphate dehydrogenase and is due to the alkylation of its essential thiol group (Harris & Waters, 1976). If the exchange takes place in the presence of these inhibitors, then a decrease in the velocity of C-2 exchange is observed. At sufficiently high concentrations of inhibitor (50-2000p~) the exchange no longer goes to completion. This behaviour can be assigned to the loss of glyceraldehyde phosphate dehydrogenase activity as the other enzymes involved are insensitive to these reagents (Webb, 1966). By using a steady-state treatment of isotope exchange (Oster et al., 1971), it can be shown that: