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Biomedical subjects

M Spencer

Publications and source records attributed to M Spencer.

At least 127 records · Page 7Linked to original sources
PubMed1973

Competitive inhibition and substrate activity of uridine diphosphate 6-deoxygalactose for Escherichia coli uridine diphosphate galactose 4-epimerase.

UDP-6-deoxygalactose inhibits the UDP-galactose 4-epimerase (EC 5.1.3.2) from Escherichia coli in a competitive manner with respect to the substrate UDP-galactose, giving K(i) 1.3x10(-3)m. As a substrate for the enzyme, it is transformed into UDP-6-deoxyglucose, although the reaction stops before equilibrium is attained. Possible causes of this behaviour are discussed.

Binding, Competitive↗
PubMed1973

Use of dextran-40 gradients for separation of pea cotyledon mitochondria into different fractions.

A crude pea (Pisum sativum L. var. Homesteader) mitochrondrial preparation was divided into two equal parts. One part was layered on a Dextran-40 step gradient, and the other on a sucrose step gradient, and they were centrifuged to obtain different bands of particles. The densities at which the particles banded and the mitochondrial respiratory activities of the particles were determined. Dextran-40 density gradient centrifugation resulted in a better separation of mitochondrial populations than did sucrose density gradient centrifugation. Separation by sucrose density gradient centrifugation may not be according to the true densities of the particles. On the other hand, the use of gradients of Dextran-40, a solute of low osmotic potential, facilitated separation of particles acording to their true densities. Such mitochondria showed better respiratory control ratio and ADP:0 values, than those isolated by sucrose density gradient centrifugation.

Journal Article↗
PubMed1973

Structural Development during Germination of Different Populations of Mitochondria from Pea Cotyledons.

The crude mitochondrial fraction from pea cotyledons can, from days 1 to 7 of germination, be separated into three fractions by sucrose density gradient centrifugation. When seeds were grown in water (control) or cycloheximide (120 micrograms per milliliter of medium) for 4 days, the originally different populations of mitochondria acquired a uniform density and separated together in band 1 (density, 1.205 grams per milliliter). The oxidative and phosphorylative activities of mitochondria obtained from 4-day-old control and 4-day-old cycloheximide-treated pea seeds were the same. However, mitochondria from pea seeds that were grown in d-threo-chloramphenicol (1.5 milligrams per milliliter of medium) or erythromycin (0.5 milligram per milliliter of medium) for 4 days separate into three bands (fully developed mitochondria in the top band [band 1] and partially developed mitochondria in the lower two bands [bands 2 and 3]). Separation patterns and oxidative and phosphorylative activities were the same for mitochondria separated from 4-day-old cotyledons treated with d-threo-chloramphenicol or erythromycin and from 1-day-old cotyledons grown in water. This indicated that these inhibitors prevented the partially developed mitochondria originally in bands 2 and 3 from developing further. In contrast, cycloheximide did not seem to interfere with the mitochondrial structural development. These results along with those obtained from the experiments on the effects of d-threo-chloramphenicol, erthromycin, and cycloheximide on (14)C-leucine incorporation into mitochondrial membrane proteins suggest that the increase in mitochondrial activity during germination may be a result of structural development (membrane synthesis) in pre-existing mitochondria.

Journal Article↗
PubMed1970

Changes in the Respiratory, Enzymatic, and Swelling and Contraction Properties of Mitochondria from Colytedons of Phaseolus vulgaris L. during Germination.

Mitochondria isolated from cotyledons of germinating wax beans (Phaseolus vulgaris L.) showed fairly good respiratory control on days 1 and 2 after planting. The respiratory control was completely lost from days 3 to 5. During this period mitochondria were shown to be very leaky, losing about 88% of their total nicotinamide adenine dinucleotide to the suspending medium in a short time. The respiratory control was partially recovered by day 7, after which it completely disappeared again. By the use of differential centrifugation, the mitochondria were divided into subfractions by sequential centrifugation: 10,000g for 5 minutes, 25,000g for 5 minutes, and 40,000g for 5 minutes. The 10,000g subfraction was responsible for the recovery of mitochondrial activity (respiratory control value, adenosine diphosphate to oxygen ratio, and rate of oxygen utilization), on day 7. Activities of succinate dehydrogenase, cytochrome oxidase, pyruvate dehydrogenase, and isocitrate dehydrogenase from different mitochondrial subfractions of aging cotyledons were determined. In general, the enzyme activities, adenosine diphosphate to oxygen ratios, and the ability of mitochondria to swell and contract followed the same pattern as for respiratory control.

Journal Article↗
PubMed1969

beta-Alanine as an Ethylene Precursor. Investigations Towards Preparation, and Properties, of a Soluble Enzyme System From a Subcellular Particulate Fraction of Bean Cotyledons.

A method is described for the preparation, from a subcellular particulate fraction of wax bean cotyledons, of a soluble enzyme system that is capable of converting beta-alanine to ethylene. In the presence of ATP, CoA, thiamine pyrophosphate, MgSO(4), and pyridoxal phosphate, ethylene production is maximum at a 0.5 mm concentration of beta-alanine. The system exhibits a pH optimum at 7.0 but when the pH is raised above 8, evolution of the volatile again increases and continues to do so up to pH 12. The enzyme system is stimulated by either NADPH or NADH; the concentration of NADPH necessary to obtain maximum activity is twice that of NADH. The requirement for a reducing agent is in agreement with the proposal that malonate semialdehyde, formed by an aminotransferase reaction from beta-alanine, is reduced to beta-hydroxypropionate. Both malonate semialdehyde and beta-hydroxypropionate are better stimulators of production of the volatile in the soluble system than is beta-alanine, and beta-hydroxypropionate is a better stimulator than malonate semialdehyde. This system is also able to incorporate tritium from tritiated water into ethylene; this supports the proposal that ethylene is formed by the decarboxylation of acrylate, the latter being formed from beta-hydroxypropionate.Experiments with both cold and labeled malonate suggest that this compound stimulates ethylene production by acting as an end product inhibitor that prevents the loss of malonate semialdehyde from the pathway. Malonate does not appear to serve as a precursor.Addition of cytoplasmic enzymes to the ;soluble system' (prepared from particulate enzymes) results in a considerable boost in ethylene production, but the specific activity (mmu1 / mg protein) is lowered from that of the particulate enzymes alone.

Journal Article↗