Biomedical subjects

R Parham

Publications and source records attributed to R Parham.

4 recordsLinked to original sources

PubMed1995

Chloroplast biogenesis 72: a [4-vinyl]chlorophyllide a reductase assay using divinyl chlorophyllide a as an exogenous substrate.

[4-Vinyl]Chlorophyllide alpha reductase (4VCR) catalyzes the conversion of 2,4-divinyl chlorophyllide alpha (DV Chlide alpha) to 2-vinyl,4-ethyl chlorophyllide alpha (MV Chlide alpha) via an NAPDH- dependent reaction. MV Childe alpha is the immediate precursor of monovinyl chlorophyll alpha in plants. In etiolated cucumber (Cucumis sativus L.) cotyledons, 4VCR is a plastidic membrane-bound enzyme. Further research on this enzyme required the development of an assay that utilizes DV Childe alpha as an exogenous substrate. Such an assay is now described. It involves conversion of exogenous DV Chlide alpha to MV Chlide alpha at high rates by etioplast membranes of cucumber, corn (Zea mays L.), and barley (Hordum vulgare L.). 4VCR exhibits high activity between 30 and 40C and in the pH range of 6.3 to 7.0. Activity is quasilinear for the first 60 s of incubation.

Chlorophyllides↗

PubMed1994

Chlorophyll a biosynthetic heterogeneity.

Chlorophyll a biosynthesis is presently interpreted in terms of two different biochemical pathways. According to one pathway, chlorophyll a is made via a single linear chain of reactions starting with divinylprotoporphyrin IX and ending with monovinylchlorophyll a. The experimental evidence for this pathway is marred by incompletely characterized intermediates that were detected in Chlorella mutants. The second pathway considers chlorophyll a to be made via multiple and parallel biosynthetic routes that result in the formation and accumulation of monovinyl- and divinylchlorophyll a chemical species. Two of these routes, namely the di/monocarboxylic monovinyl and divinyl routes, are responsible for the biosynthesis of most of the chlorophyll a in green plants. The experimental evidence for these two routes consists of: (a) the detection and spectroscopic characterization of intermediates and end products; (b) the demonstration of precursor-product relationships between various intermediates in vivo and in vitro; and (c) the detection of 4-vinylreductases that appear to be mainly responsible for the observed biosynthetic heterogeneity. The biological significance of chlorophyll a biosynthetic heterogeneity is becoming better understood. On the basis of the prevalence of the di/monocarboxylic monovinyl-and divinylchlorophyll a biosynthetic routes, green plants have been classified into three different greening groups. It now appears that the major chlorophylls in the euphotic zone of tropical waters are divinylchlorophyll a and b. It also appears that the di/monocarboxylic monovinyl and divinyl biosynthetic routes lead to the formation of different pigment proteins in different greening groups of plants, and that the more highly evolved monovinylchlorophyll a biosynthetic route is associated with higher field productivity in wheat.

Chlorella↗

PubMed1992

Chloroplast biogenesis: [4-vinyl] chlorophyllide a reductase is a divinyl chlorophyllide a-specific, NADPH-dependent enzyme.

Some properties of [4-vinyl] chlorophyllide a reductase are described. This enzyme converts divinyl chlorophyllide a to monovinyl chlorophyllide a. The latter is the immediate precursor of monovinyl chlorophyll a, the main chlorophyll in green plants. [4-Vinyl] chlorophyllide a reductase plays an important role in daylight during the conversion of divinyl protochlorophyllide a to monovinyl chlorophyll a. [4-Vinyl] chlorophyllide a reductase was detected in isolated plastid membranes. Its activity is strictly dependent on the availability of NADPH. Other reductants such as NADH and GSH were ineffective. The enzyme appears to be specific for divinyl chlorophyllide a, and it does not reduce divinyl protochlorophyllide a to monovinyl protochlorophyllide a. The conversion of divinyl protochlorophyllide a to monovinyl protochlorophyllide a has been demonstrated in barley and cucumber etiochloroplasts and appears to be catalyzed by a [4-vinyl] protochlorophyllide a reductase [Tripathy, B.C., & Rebeiz, C.A. (1988) Plant Physiol. 87, 89-94]. On the basis of reductant requirements and substrate specificity, it is possible that two different 4-vinyl reductases may be involved in the reduction of divinyl protochlorophyllide a and divinyl chlorophyllide a to their respective 4-ethyl analogues.

Chlorophyll↗

PubMed1992

Chloroplast Biogenesis 65 : Enzymic Conversion of Protoporphyrin IX to Mg-Protoporphyrin IX in a Subplastidic Membrane Fraction of Cucumber Etiochloroplasts.

The preparation from Percoll-purified cucumber (Cucumis sativus)etiochloroplasts of a subplastidic membrane fraction that is capable of high rates of Mg insertion into protoporphyrin IX is described. The plastid stroma was inactive when used either alone or in combination with the membrane fraction. Successful preparation of the subplastidic membrane fraction required that Mg-protoporphyrin chelatase was first stabilized by its substrate. This was achieved by lysing Percoll-purified plastids in a fortified hypotonic medium containing protoporphyrin IX prior to ultracentrifugation and separation of the stroma from the plastid membranes. Protoporphyrin IX became membrane bound. Other additives needed for enzyme activity fell into two groups: (a) those needed for enzyme stabilization during membrane preparation and (b) those involved in the primary mechanism of Mg insertion into protoporphyrin IX. Ethylenediaminetetraacetate belonged to the first group, magnesium belonged to the second group, and ATP belonged to both groups.

Journal Article↗