Polyamine catabolism in higher plants: Characterization of pyrroline dehydrogenase

Abstract

Both mono-and dicotyledonous species catabolize putrescine to -r-aminobutyric acid (GABA), but by two different pathways. GABA is the major labeled product in pea shoots and oat leaves fed with a 2-4h pulse of [1,4-14C]-putrescine (Put) or [1,4-tetramethylene-~*C]-spermidine (Spd), respectively. In the presence of 1-10 ~M gabaculine, a specific inhibitor of GABA:pyruvate-transaminase, the label appearing in GABA increases 2 to 7-fold, which indicates that the transamination reaction is a major fate of GABA formed from Put or Spd in vivo. The conversions to GABA were demonstrated in vitro in coupled assays involving diamine oxidase from pea or polyamine oxidase from oat, and pyrroline dehydrogenase (PYRR-DH). The latter enzyme from either pea or oat is strictly NAD-dependent and is specific for pyrroline. The optimal temperature (40-45 °C) and pH (7.5-8.0) are similar to those of bacterial PYRR-DH. In all cases the enzyme was inhibited by the NAD analogs thionicotinamide and aminopyridine dinucleotide (0.1-1.0 mM). In addition to pea and oat, PYRR-DH was also detected in corn, barley, soybean and broadbean. Di-and polyamine oxidase are released by enzymes which degrade the cell wall, while PYRR-DH remains associated with the protoplast.

Polyamine Metabolism in Ripening Tomato Fruit

“…The neutral fraction also contained many peaks on GC (retention times ranging from 20-30 min) and their spectra very closely matched those of various sugars (spectra not shown). The conversion of Ptc into GABA suggests that a pathway involving a diamine oxidase may be operative in the tomato pericarp tissue: diamine oxidase produces y-aminobutyraldehyde (which spontaneously interconverts into pyrroline) which upon further oxidation results in GABA (7). This possibility was tested by application of AG, an inhibitor of diamine oxidase (3,7,21).…”

“…The conversion of Ptc into GABA suggests that a pathway involving a diamine oxidase may be operative in the tomato pericarp tissue: diamine oxidase produces y-aminobutyraldehyde (which spontaneously interconverts into pyrroline) which upon further oxidation results in GABA (7). This possibility was tested by application of AG, an inhibitor of diamine oxidase (3,7,21). Treatment with AG produced a small but significant inhibition of Ptc breakdown with a concomitant reduction in the formation of GABA, Glu, sugars, and organic acids (Table III).…”

“…Whether Glu, sugars, and organic acids were derived directly from Ptc, or whether GABA was an intermediate in their production was probed by using GCU (an irreversible inhibitor ofGABA aminotransferase; 7,8,12). In the presence of GCU, the GABA fraction accumulated twice as much radioactivity as in its absence, with a concomitant decrease by several-fold in the label appearing in Glu, and sugar and organic acid fractions (Table IV).…”

“…It is quite possible that pyrroline was produced but rapidly converted to GABA. Aminoguanidine, which inhibits Ptc breakdown in some plants (3,7) and animals (21), produced a small inhibition of Ptc breakdown in the pericarp tissue (Table III). It is likely that either AG did not fully get to the active site or the tomato diamine oxidase is somewhat different from that found in other plants.…”

“…Also, pea shoots and oat leaves, respectively, were shown to oxidatively metabolize Ptc and Spd to GABA (7).…”