NADH/NADPH--cytochrome c reductase assay: interference by nonenzymatic cytochrome c reducing activity in plant extracts

Oxidation of ethylene by cotyledon extracts from Vicia faba L. : Cofactor requirements and kinetics

Improved rates of ethylene oxidation by cell-free preparations from cotyledons of Vicia faba L. have been obtained using cryogenic storage techniques and by developing a method for the hydrolysis of ethylene oxide. Gel permeation chromatography showed that a low-molecular-size fraction was required for activity; accordingly, the kinetics of ethylene oxidation in the presence of this fraction were studied. Reduced pyridine nucleotides could substitute for the low-molecular-size fraction. Activity under a nitrogen atmosphere was 60% lower than in air. The need for reduced nicotinamide adenine dinucleotide phosphate (NADPH) and oxygen indicated that the enzyme might be a mixed-function oxidase. Using sufficient NADPH to approach saturation, the apparent Michaelis constant (K m) for ethylene was 1.94±0.38 · 10(-8) M (aqueous phase), and when ethylene was saturating, the K m for NADPH was 3.7 · 10(-5) M. Carbon monoxide was found to inhibit by competing with ethylene, and the inhibitor constant was 5.97 · 10(-7) M in solution. In the presence of excess ethylene and NADPH, activity was highest in phosphate-buffered medium pH 7.9. The bulk of the activity was found in a microsomal fraction.

Synthesis of prenyl lipids in cells of spinach leaf. Compartmentation of enzymes for formation of isopentenyl diphosphate

Purified spinach chloroplasts incorporate [1-14C]isopentenyl diphosphate into prenyl lipids in high yields. The immediate biosynthetic precursors of isopentenyl diphosphate (hydroxymethylglutaryl-CoA, mevalonate, mevalonate-5-phosphate, mevalonate-5-diphosphate), on the other hand, are not accepted as substrates and the corresponding enzymes hydroxymethylglutaryl-CoA reductase, mevalonate kinase, phosphomevalonate kinase, and diphosphomevalonate decarboxylase are not present in the organelles. These enzymes can only be detected in a membrane-bound form at the endoplasmic reticulum (hydroxymethylglutaryl-CoA reductase) and as soluble activities in the cytoplasm. The concept is developed that isopentenyl diphosphate is formed in the cytoplasm as a 'central intermediate' and is distributed then to other cellular compartments (endoplasmic reticulum, plastids, mitochondria) for further biosynthetic utilization.

Quinone reductases of higher plants

NAD(P)H: quinone oxidoreductase (DT-diaphorase) was detected in 100000 x g supernatant fractions of extracts of a wide variety of higher plants. Smaller amounts were also found in microsomes and chloroplast fractions. The enzyme was partially purified from soluble extracts of several plants and the quinone reductase from Catharanthus roseus was enriched 25-fold. Plant quinone reductases have molecular weights in the range of 38000-53000 as determined by gel filtration. The plant enzyme is far less sensitive to dicoumarol than its mammalian counterpart and it is inhibited by superoxide dismutase. Quinone reductase is capable of reducing simple p-benzoquinone and naphthoquinone including vitamins K3 and K1. These results indicate that, although the plant enzyme exhibits a similar substrate specificity, it is distinguishable from mammalian DT-diaphorase particularly with respect to its mechanism of reduction.