Pentose cycle activity in cell-free extracts of tobacco leaves and seedlings

Abstract 1. 1. Evidence is presented for pentose cycle activity in extracts of tobacco leaves and seedlings; a second pathway is also indicated. All oxidations studied are TPN specific. 2. 2. The presence in our extracts of a TPN-specific triosephosphate dehydrogenase is suggested; this enzyme is active in the absence of phosphate and is not stimulated by arsenate. 3. 3. The sulfhydryl nature of tobacco leaf G-6-P dehydrogenase is indicated by PCMB inhibition and the reversal of this inhibition by cysteine. 4. 4. The oxidation of G-6-P by tobacco leaf extracts is inhibited by ATP; the inhibition is decreased by about 50% if the enzyme is preincubated with KF. 5. 5. With our extracts, FMN is required for maximal stimulation of oxygen uptake by pentose cycle intermediates. Although not required for maximal stimulation of oxygen uptake by G-6-P, Mg is required for the liberation of CO 2 from G-6-P and 6-PGA. 6. 6. With some enzyme preparations, the stimulation of oxygen uptake by G-6-P and HDP is partially inhibited by cyanide; this inhibition can be relieved by FMN. 7. 7. Pentose cycle activity is found to be highest in extracts of tobacco seedlings and young leaves; extracts of leaves at, or below, midstalk position of the mature plants are devoid of pentose cycle activity. 8. 8. Where possible, a comparison is made with studies of pentose cycle activity reported for pea leaf extracts.