Intracellular location and possible function of malic dehydrogenase isozymes from young maize root tissue.

It is known that there is a separation, i.e., compartmentation of organic acids in young maize tissue (11) and that the organic acid prodtucts (specifically malic acid) of CO, fixation are not in complete equilibriuim with mitochondrial pools (9, 10). Thus, there are at least 2 essentially separate metabolic pools of organic acids. Previous work with corn root tips led tus to postulate a metabolic sequience in which reductive power from NADH is transferred to NADP (14). The sequential enzymes are P-enolpyruvate carboxylase, NAD malic dehydrogenase, and NADP malic dehydrogenase (i.e., the malic enzyme). Although P-enolpyruvate carboxylase and the malic enzyme are evidently enzymes of the soluble or cytoplasmic fraction (as defined in this report, not sedimented in stucrose at 10,000 X g after 30 min), NAD malic dehydrogenase is known from both soluble and particulate fractions. For example, Price and Thimann (13) showed malic dehydrogenase activitv from mitochondrial and soluble fractions of plant tissule. Kaplan and Ciotti (8) demonstrated differences between the 2 forms with the use of pyridine nucleotide analog ratios. Davies (3) separated 2 malic dehydrogenase bands on DEAE cellulose colutmns from pea epicotyl tissue and Boser and Pawelke (2) found 2 peaks during a coluimn separation from potato tubers. We were prompted to investigate the malic dehydrogenase isozymes (12) in young corn tissuie (Bturpee's Golden Cross Bantam) with the hypothesis that there would be at least 2 isozymes (with different metabolic fuinctions!) compartmentalized in (1) the cytoplasmic fraction and (2) the mitochondrial fraction. Maize root tips were grown on moist filter paper at 250 for 3 days prior to harvesting the distal 2 cm. The total NAI) malic dehydrogenase fraction was obtained by freezing the tisstue in No, grinding to a fine powder, sutspending in an equial volume of (w/v) of 0.01 m, pH 7.4 Tris buiffer, and centrifuging at