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Biomedical subjects

A de Waal

Publications and source records attributed to A de Waal.

7 recordsLinked to original sources
PubMed1992

Coordinated regulation of two indole alkaloid biosynthetic genes from Catharanthus roseus by auxin and elicitors.

Catharanthus roseus (periwinkle) produces a wide range of terpenoid indole alkaloids, including several pharmaceutically important compounds, from the intermediate strictosidine. The complete mRNA sequence for the enzyme strictosidine synthase (SSS) was determined. Comparison of the primary structure of the encoded protein with the amino-terminal sequence of purified SSS indicated the presence of a signal peptide of 31 amino acids in the putative primary translation product. SSS is encoded by a single-copy gene indicating that isoenzymes reported by others are formed post-translationally from a single precursor. The sss gene and the tryptophan decarboxylase gene (tdc), encoding another enzyme essential for indole alkaloid biosynthesis, are coordinately regulated. In plants steady-state mRNA levels are highest in roots. In cell suspension cultures the genes are rapidly down-regulated by auxin. In contrast, both genes are strongly induced by fungal elicitors such as Pythium aphanidermatum culture filtrate or yeast extract. Induction is a rapid, transcriptional event occurring independent of de novo protein synthesis. These results show that a first important regulatory step in the complex process leading to indole alkaloid accumulation in C. roseus suspension cells is transcription of the biosynthetic genes.

Alkaloids↗
PubMed1991

Magainins affect respiratory control, membrane potential and motility of hamster spermatozoa.

The hypothesis was tested that the magainin peptides, known to compromise bacterial and mitochondrial energetics, are highly active against spermatozoa. A mixture of magainin A and PGLa (1:1) caused a 50% reduction in motility of hamster spermatozoa at 4 micrograms/ml total peptide concentration. All motility was lost at 8 micrograms/ml. At this concentration, respiratory control was released and respiration in the presence of uncoupler was inhibited. Uptake of the lipophilic cation tetraphenyl phosphonium was largely abolished by addition of magainin A and PGLa showed synergism with respect to release of respiratory control.

Amino Acid Sequence↗
PubMed1991

The kinetics of the hydroxylation of procollagen by prolyl 4-hydroxylase. Proposal for a processive mechanism of binding of the dimeric hydroxylating enzyme in relation to the high kcat/Km ratio and a conformational requirement for hydroxylation of -X-Pro-Gly- sequences.

Prolyl 4-hydroxylase modifies only approx. 5% of the hydroxylatable prolyl residues in procollagen at a relatively high rate, after which the rate of further hydroxylation rapidly decreases. This suggests that the probability to exist in a defined hydroxylation-committed conformation differs between the numerous -X-Pro-Gly- sequences in the substrate. The enzyme reaction is characterized by the unusually high kcat/Km ratio of 3 x 10(9) M-1 s-1. To explain these kinetic features, an extremely high second-order rate constant for the association of enzyme and the subset of rapidly hydroxylated prolyl residues has to be assumed. A two-step mechanism is proposed in which diffusional constraints on the rate of association of prolyl 4-hydroxylase with hydroxylatable prolyl residues can be overcome. Upon encountering a random coil pro-alpha chain, the dimeric enzyme is first 'aspecifically' bound, followed by rapid transfers between different segments of the flexible peptide substrate via fast transitions between 'aspecific' single and double bound intermediate states. The rate of the second step, the productive (specific) binding of hydroxylation-committed -X-Pro-Gly- sequence to the active site, can be enhanced significantly by such an, in essence, 'one-dimensional' search. This processive mechanisms of binding does not necessarily imply many hydroxylation reactions during one encounter between enzyme and a peptide with several substrate sites as suggested previously in a slightly different model (De Waal, A. and De Jong, L. (1988) Biochemistry 27, 150-155).

Amino Acid Sequence↗
PubMed1991

Evaluation of an electronic device used as a quick screening method to detect changes in the red cell content of the blood in horses participating in endurance trail rides.

Dehydration, to varying degrees, will inevitably occur in horses participating in endurance trail rides. This water loss is directly related to the amount of body water lost through evaporative cooling, that in turn being related to the amount and rate of work performed, and to the environmental temperature and humidity. An electronic apparatus that can substitute for the conventional and time-consuming venous haematocrit was developed to measure the animals' hydration status. The mechanism of this electrometer is based upon the relationship between the red cell content and the electrical conductivity of the blood. The electrometer reading gave a significant correlation (p less than 0.001) with the venous haematocrit, thus indicating that the electrometer is a reliable apparatus for determining the animals' hydration status within seconds of venepuncture.

Animals↗
PubMed1990

Constraints on weaning: evidence from Ethiopia and Sudan.

Evidence on infant weaning processes provided by field research in Addis Ababa, Ethiopia, and among a refugee population of Eritreans in the Sudan is presented. The study in Addis Ababa, where households were economically disadvantaged but the situation was nutritionally stable, allowed the identification of factors constraining weaning, which includes both the introduction of supplementary foods and the termination of breast-feeding. While the timing of each aspect of weaning was the outcome of the interaction between mother and infant factors, 'infant-centred' factors were more important in the commencement and 'mother-centred' factors in the completion of weaning. The refugee population had in normal times a similar weaning regime, but under the unstable conditions of flight and life in a refugee camp the process had been significantly altered. Investigation of the factors which have led to altered weaning processes elaborates and confirms the model derived from the Addis Ababa study.

Breast Feeding↗
PubMed1978

Electrocardiographic and biochemical studies on marathon runners.

The electrocardiographic findings and certain biochemical observations in 48 male runners before and after participation in the Comrades Marathon race are presented. In 21 of the 48 runners electrocardiographic changes consistent with early ischaemic heart disease could be demonstrated before the race. In 2 runners who had normal tracings before the race, abnormalities could be shown after the race. The total serum creatine kinase activity was markedly elevated after the race in the majority of the 23 runners in whom this was measured. In 11 of these subjects, the Marsh-Bender (MB) fraction could be demonstrated after the race. It is concluded that an elevated MB fraction is probably not a reliable index of myocardial damage after strenuous exercise.

Adult↗