Thesis (doctoral)--Lund University, 1997. / Added t.p. with thesis statement inserted.

Thesis (doctoral)--Lund University, 1997. / Added t.p. with thesis statement inserted.

No description available.

NAD-Dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase is a mitochondrial folate-dependent enzyme apparently expressed only in transformed mammalian cells and tissues containing undifferentiated cells. The cloning of both the cDNA and the gene encoding this bifunctional enzyme have led to a better understanding of the regulation of its expression in normal and transformed cells. Southern blot analyses revealed that the gene has a similar organization in normal and transformed cells and no major rearrangement or amplification were detected. The gene encodes two messenger RNAs which differ only by the length of their 3$ sp prime$-untranslated region and are derived by the use of alternative polyadenylation signals. The detection of transcripts in transformed cells and not in normal adult tissues suggests that the gene is primarily regulated at the transcriptional level. The promoter region of the gene contains several putative cis-regulatory elements which may have an important role in the differential expression of this folate-dependent enzyme.

No description available.

The kinetic relationship between the activities of multifunctional enzymes from folate-mediated one-carbon metabolism were examined. Formyltetrahydrofolate dehydrogenase catalyzes the main disposal reaction for excess one-carbon units produced in the liver. The enzyme, which catalyzes also a hydrolytic reaction, was purified from porcine liver and a radioactive assay was developed to measure both activities simultaneously. These and other kinetic measurements established that the dehydrogenase and hydrolase are kinetically independent activities of a single type of polypeptide. NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase is found in all transformed mammalian cells and catalyzes sequential reactions with channeling of the metabolic intermediate. However, these activities are kinetically independent in contrast to similar activities of the NADP-dependent trifunctional enzyme found in all eukaryotic cells. These properties explain the observation that only the NAD-dependent enzyme conjugate can catalyze the conversion of formyl- to methylenetetrahydrofolate in vitro.

Proefschrift--Groningen. / Summary in English. Bibliography: p. [92]-105.

No description available.

Folic acid is present in foods in many different forms. Concentrations of N⁵-methyltetrahydrofolic acid (one form of folate) were determined by a radioassay and the Lactobacillus casei microbiological assay methods. The coefficient of determination for samples analyzed by both methods was 0.86 but the radioassay method was found to be faster, simpler and more accurate than the microbiological method. The milk folate binder and L. casei were found to react differently to pteroylglutamic acid (another form of folate) compared to N⁵-methyltetrahydrofolic acid. The two methods of assay were found to be suitable only for measuring the form of folate used to construct the standard curve for the assay. It was found that neither method could be used to quantify- folate levels in foods accurately. Thus, at best, presently available data on food folate levels can only be compared to other values obtained by an identical method. Also folate concentration values presently available may not necessarily be a reflection of their nutritional significance to man. The radioassay method was used for the measurement of N⁵-methyltetrahydrofolic acid degradation under various conditions. It was found that N⁵-methyltetrahydrofolic acid degradation in the presence of an unlimited oxygen supply could be described as a pseudo first order reaction. Rate constants for the reaction were found to increase as the temperature of the reaction increased and were described in terms of the Arrhenius equation. The overall reaction appeared to be second order in the presence of a limited oxygen supply. The presence of mercaptoethanol in the assay system delayed the beginning of the N⁵-methyltetrahydrofolic acid degradation reaction. The presence of ascorbic acid in the N⁵-methyltetrahydrofolic acid/buffer solution also delayed the start of the reaction. The reaction rate constants were not altered by the presence of the two reducing agents, however. The degradation of N⁵-methyltetrahydrofolic acid was therefore concluded to be an oxidation reaction. The degradation product was identified as N⁵-methyldihydrofolic acid by ultraviolet spectroscopy. The results of this research implicate the importance of reducing agents in foods containing folate which are subjected to heat processing. Experiments with food materials containing ascorbic acid and exposed to high temperatures indicated that the degradation of free folate was delayed by the presence of the reducing agent. / Land and Food Systems, Faculty of / Graduate

No description available.