Mechanism of action of 1,25-dihydroxycholecalciferol on intestinal calcium transport

The prior administration of actinomycin D prevents the metabolism of [(3)H]25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, a metabolite of vitamin D(3) that is effective in the stimulation of intestinal calcium transport. In this paper, the question of whether the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is sensitive to actinomycin D was examined. While the response of intestinal transport to physiological amounts of 25-hydroxycholecalciferol is blocked by actinomycin D, the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is insensitive to the antibiotic. These results suggest that 1,25-dihydroxycholecalciferol, or a further metabolite thereof, is the metabolically active form of vitamin D in the intestine, that it functions by a process not involving transcription of DNA, and that the step sensitive to actinomycin D in the action of vitamin D on the intestine does not occur in the intestine, but is the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidney.

Inhibition of the metabolism of 25-hydroxycholecalciferol by actinomycin D and cycloheximide

Actinomycin D (or cycloheximide) administered prior to radioactive 25-hydroxycholecalciferol blocks the metabolism of 25-hydroxycholecalciferol to polar metabolites that accumulate in intestinal tissue, while it does not prevent the 25-hydroxylation of vitamin D(3) in the liver. Actinomycin D given after radioactive 25-hydroxycholecalciferol does not inhibit 25-hydroxycholecalciferol metabolism. These results indicate that 25-hydroxycholecalciferol must interact with the nuclei of cells to bring about the production of an enzyme(s) that converts it to its polar metabolites.

Enzymic Mechanism of Starch Breakdown in Germinating Rice Seeds: III. alpha-Amylase Isozymes

The formation of amylase isozymes in germinating rice (Oryza sativa) seeds was studied by isoelectric focusing on polyacrylamide gel disc electrophoresis. Time sequence comparisons of the amylase zymogram were made between extracts from gibberellic acid-treated embryoless and embryo-attached half-endosperm of rice seeds. In both cases, 4 major and 9 to 10 minor isozyme bands were detectable at the maximal stage of the enzyme induction. However, in the embryo-attached half-seeds, bands started to diminish after the 5th day of incubation, in agreement with the results of time sequence analyses of enzyme activities. Nearly identical patterns of amylase isozyme bands on a polyacrylamide gel disc electrophoresis in combination with isoelectric focusing indicate the intrinsic role of gibberellic acid in the starch breakdown in germinating rice seeds. We tentatively assign the newly synthesized enzymes to be alpha-amylases based on experimental results concerning the lability of the preparation on a prolonged treatment at pH 3.3 and the stability on heat treatment for 15 minutes at 70 C.

alpha-Amylase Isozymes in Gibberellic Acid-treated Barley Half-seeds

The presence of multiple forms of alpha-amylase in gibberellic acid-treated embryoless barley half-seeds was demonstrated by separation on diethylaminoethyl-Sephadex and isoelectric focusing polyacrylamide gel disc electrophoresis. Two major alpha-amylase fractions (A and B), each consisting of two to three isozyme components, were purified. alpha-Amylase fractions A and B were distinguishable in their reaction patterns. The optimal pH of fraction A alpha-amylase was found to reside in the acidic side (pH 5.0), as was determined by analyzing the reducing sugars formed as well as the paper chromatographic detection of reaction products. At neutral pH, 6.9, fraction A exhibited weak amylolytic activity in forming maltose. The alpha-amylase activity in fraction A was markedly stimulated by heat treatment (70 C/15 minutes). Fraction B, constituting a major part of amylases in the endosperm extract, was also found to be composed of alpha-amylase, as evidenced by the loss of enzyme activity upon allowing fractions A and B to stand at pH 3.3 for a prolonged period. The possible physiological function of the two different types of alpha-amylase in the carbohydrate breakdown of barley seeds is discussed.

Studies on inhibition of intestinal absorption of radioactive strontium. IX. Relationship between biological activity and electron microscopic appearance of alginic acid components

The inhibitory action of alginate on intestinal absorption of radioactive strontium was investigated in order to correlate the biological activity with the chemical composition. Alginate from Laminaria hyperborea was partially hydrolyzed with oxalic acid and the degradation products were fractionated into polymannuronic and polyguluronic acid. The activity of these products was assessed biologically in rats and morphologically by electron microscopy. Sodium polymannuronate was found to be less effective than sodium polyguluronate in preventing absorption of radiostrontium. The inhibition of absorption of radio-calcium was low and not affected by hydrolysis or fractionation. When dried from dilute aqueous solutions, the polymannuronate retained the original helical structure of alginate, while the polyguluronate showed a strong tendency to coagulate, forming granules. The variation in the biological activity was attributed to the morphological differences between these alginic acid components and it is suggested that the degree of uncoiling of the polyguluronate chain in water is greater than that of the polymannuronate chain, thus making the carboxylate ions more accessible to strontium.