Mary W. Loewus

Biosynthesis and metabolism of ascorbic acid in plants

The biosynthesis of L‐ascorbic acid in plants differs from that encountered in ascorbic acid‐synthesizing animals. Enzymic details are sparse, but in vivo studies with tracers clearly establish the stereochemical detail of both processes. Examples of each process are found in separate classes of algae. Plants utilize L‐ascorbic acid as the carbon source for the biosynthesis of two important plant acids, oxalic acid and L‐tartaric acid. Here, cleavage of L‐ascorbic acid between carbons 2 and 3 releases the 2 and 4 carbon intermediates. A second L‐tartaric acid‐synthesizing process peculiar to vitaceous plants, i.e., grape, cleaves ascorbic acid between carbons 4 and 5. The physiological significance of these metabolic interconversions is discussed. Other metabolic processes such as the oxidation/reduction properties of L‐ascorbic acid are also considered.

myo-Inositol: Biosynthesis and Metabolism

Publisher Summary This chapter discusses the biosynthesis and metabolism in myo -inositol. The awareness about the role of myo -inositol in carbohydrate metabolism emerges not only from an appreciation of the biosynthetic origin and metabolic fate of myo -inositol in higher plants, but also from the realization that hitherto unknown or poorly understood biochemical processes may at present be reconsidered in the light of new observations. Virtually, nothing is known concerning the regulation of myo -inositol biosynthesis, myo -Inositol-1-P synthase appears to be the only enzyme involved in myo-inositol biosynthesis, plant or animal. Conversion of myo -inositol-1-P to inositol must be studied more thoroughly as the free inositol pool is a very important nutritional reserve to the newly germinating seed or the germinating pollen grain. In regard to the metabolism of myo -inositol, there is very little information available regarding the fate of myo -inositol as derived from the reserves of phytic acid in the germinating seed. The information gathered suggests that the core substance of phytic acid is every bit as useful to the new plant as the phosphate it sheds.

Partial purification and study of pollen glucuronokinase.

Abstract Glucuronokinase was purified 31-fold from pollen of Lilium longiflorum . The enzyme was inhibited by its product, α- d -glucuronate-1-P, and by UDP- d -glucuronate, and these compounds were competitive inhibitors. Inhibitor constants were 0.18 m m for d -glucuronate-1-P and 0.55 m m for UDP- d -glucuronate. These effects may have regulatory significance; both inhibitors are intermediates in the pathway by which plant cells convert myo -inositol into cell wall uronides and pentoses, and glucuronokinase is a likely step for regulation in this pathway. The enzyme exhibited considerable specificity concerning inhibitors, and an additional 22 compounds were not inhibitory. These included uronic acids other than d -glucuronate and compounds related structurally or metabolically to d -glucuronate.

The C-5 hydrogen isotope-effect in myo-inositol 1-phosphate synthase as evidence for the myo-inositol oxidation-pathway

The hydrogen isotope-effect that occurs in vitro during myo-inositol1-phosphate synthase-catalyzed conversion of D-[5-3H]glucose 6-phosphate into myo[2-3H]inositol 1-phosphate has been used to compare the functional role of the nucleotide sugar oxidation-pathway with that of the myo-inositol oxidation-pathway in germinating lily pollen. Results reveal a significant difference between the 3H/14C ratios of glucosyl and galactosyluronic residues from pectinase-amyloglucosidase hydrolyzates of the 70% ethanol-insoluble fraction of D-[5-3H, 1-14C]glucose-labeled, germinating lily pollen. This isotope effect at C-5 of D-glucose that occurred during its conversion into D-galactosyluronic residues of pectic substance is not explained by loss of 3H when UDP-D-[5-3H, 1-14C]glucose is oxidized by UDP-D-glucose dehydrogenase from germinating lily pollen. The evidence obtained from this study favors a functional role for the myo-inositol oxidation-pathway during in vivo conversion of glucose into galactosyluronic residues of pectin in germinating lily pollen.

myo-Inositol-1-phosphate synthase from pine pollen: sulfhydryl involvement at the active site.

myo-Inositol-1-phosphate synthase [EC 5.5.1.4; 1L-myo-inositol-1-phosphate lyase, (isomerizing)] from Pinus ponderosa pollen has been partially purified and characterized. It has a pH optimum between 7.25 and 7.75. The km for D-glucose 6-phosphate (NAD+ constant at 1 mM) is 0.33 mM. Inhibition by p-chloromercuribenzoate and N-ethylmaleimide, and partial protection against this inhibition by D-glucose 6-phosphate in the presence of NAD+, suggests that there is sulfhydryl group involvement at the substrate binding site.

Studies on myo-inositol-1-phosphate synthase from Lilium longiflorum pollen, Neurospora crassa and bovine testis Further evidence that a classical aldolase step is not utilized

Abstract myo-Inositol-1-phosphate synthase (1 l -myo-inositol-1-phosphate lyase (isomerizing), EC 5.5.1.4.) preparations purified from the pollen of Lilium longiflorum, and from Neurospora crassa have been incubated with d -[5- 18 O]-glucose -6-P and the myo-inositol which was formed was analyzed for retention of 18O. In each case, the isotope of oxygen was incorporated into the inositol without loss. If a Schiff base had formed at the 5-position of the [ 18 O]glucose -6-P , the isotope should have been released to the incubation medium. Supporting evidence that no Schiff base is formed at the 5-position, or at any other position, was obtained by incubation of the enzymes in media enriched with H218O. If a Schiff base were formed and hydrolyzed, the regenerated carbonyl should become enriched to the level of the medium as reflected by the isotope content of the inositol product. In no case did the product inositol have this degree of enrichment. These data exclude consideration of a Class I aldolase mechanism for these enzymes. The Lilium enzyme is unaffected by EDTA to a concentration of 100 mM. The bovine enzyme is similarly uninhibited by EDTA to a concentration of 50 mM. The Neurospora enzyme has previously been shown to be inhibited by these levels of EDTA and to be activated 2-fold by 2 mM Mg2+, however, extensive dialysis against EDTA does not eliminate the metal independent activity of the enzyme. In the present study, we have found that divalent metals show a range of stimulation/inhibition with the Lilium and the bovine enzymes, however, neither enzyme is dependent on the metals. Thus, we suggest that these enzymes are not Class II aldolase enzymes either. It is, therefore, possible that the myo-inositol-1-phosphate synthase from these species has an aldol step in the enzymatic pathway which is of neither classical aldolase type.

ASPECTS OF myo -INOSITOL METABOLISM AND BIOSYNTHESIS IN HIGHER PLANTS

SUMMARY: Germinated lily pollen contains a glucogenic process for the conversion of myo-inositol to starch over the myo-inositol oxidation pathway and the pentose phosphate pathway. The key intermediates between these two pathways are UDP-D-xylose and free D-xylose which allow elongating pollen tubes to utilize exogenous L-arabinose and D-xylose as carbon sources for the synthesis of glucose. Evidence is presented for the functional role of the myo-inositol oxidation pathway in pectin biosynthesis. Evidence is also presented to support the view that the cyclization of D-glucose 6-phosphate to myo-inositol 1-phosphate includes a significant isotope rate effect at carbon 5 of the substrate.

1l-myo-inositol-1-phosphate synthase in serum from Hevea latex☆

Abstract Serum from the latex of Hevea brasiliensis Muell. Arg. contains 1 l - myo -inositol 1-phosphate synthase (MI-1-P synthase) (EC 5.5.1.4). An endogenous inhibitor of this enzyme is present in the crude serum but can be separated from the synthase on a Sephadex G-200 gel column. The activity of partially purified synthase from Hevea latex approximates that found in pollen extracts from Lilium longiflorum .