Some observations on the vitamin B12 content of rat's milk

1. Vitamin B12 activity in the milk of rats on various diets was measured by microbiological assay at intervals throughout lactation. The concentration of sodium and calcium in the milk was also determined.

2. At all stages of lactation the milk of rats fed on commercial rat cake contained much more vitamin B12 activity than that of rats receiving a semi-synthetic diet containing at least as much vitamin B12 as the rat cake. The milk of the latter group of animals contained more vitamin B12 than that from rats fed on the same diet without added cyanocobalamin. The levels of sodium and calcium were much less variable than those of vitamin B12 and the changes did not appear to be related to differences in diet, stage of lactation or litter size.

The effect of antibiotics on rats receiving a vitamin B12-deficient diet

1. Rats fed on a vitamin B12-deficient or -supplemented diet were given either neomycin or a mixture of streptomycin and erythromycin by mouth for between 7 and 15 d. The urinary excretion of methylmalonic acid and the levels of vitamin B12 in plasma and tissues and of acetic and propionic acids in caecal contents were measured.

2. Both treatments caused prompt reduction of methylmalonate excretion in the deficient rats. This was apparently due to depression of the production of some precursor of methyl-malonic acid, probably propionate, rather than to an immediate effect on vitamin B12 nutrition.

3. After withdrawal of the antibiotics, neomycin-treated vitamin B12-deficient rats appeared to become partly repleted in vitamin B12, but the change in the vitamin B12 status of those which had received streptomycin and erythromycin was much smaller.

Gluconeogenesis from propionate in kidney and liver of the vitamin B12-deficient rat

1. Kidney-cortex slices and the perfused livers of vitamin B(12)-deficient rats removed propionate from the incubation and perfusion media at 33 and 17% respectively of the rates found with tissues from rats receiving either a normal or a vitamin B(12)-supplemented diet. There was a corresponding fall in the rates of glucose synthesis from propionate in both tissues. 2. The addition of hydroxocobalamin or dimethylbenzimidazolylcobamide coenzyme to kidney-cortex slices from vitamin B(12)-deficient rats in vitro failed to restore the normal capacity for propionate metabolism. 3. Although the vitamin B(12)-deficient rat excretes measurable amounts of methylmalonate, no methylmalonate production could be detected (probably because of the low sensitivity of the method) when kidney-cortex slices or livers from deficient rats were incubated or perfused with propionate. 4. The addition of methylmalonate (5mm) to kidney-cortex slices from rats fed on a normal diet inhibited gluconeogenesis from propionate by 25%. 5. Methylmalonate formation is normally only a small fraction of the flux through methylmalonyl-CoA. This fraction increases in vitamin B(12)-deficient tissues (as shown by the urinary excretion of methylmalonate) presumably because the concentration of methylmalonyl-CoA rises as a result of low activity of methylmalonyl-CoA mutase (EC 5.4.99.2). Slow removal of methylmalonyl-CoA might depress propionate uptake owing to the reversibility of the steps leading to methylmalonyl-CoA formation.

Dietary depletion of vitamin B12 and the excretion of methylmalonic acid in the rat

1. A semi-synthetic vitamin B12-deficient diet, based on soya flour, is described. When supplemented with cydnocobalamin the diet appeared to be adequate for growth and reproduction in the rat.

2. Compared with their litter-mates on the supplemented diet, rats fed on the deficient diet from weaning showed reduced levels of vitamin B12activity in the plasma and tissues, but their growth rates were unaffected unless they were bred from mothers that had been given the deficient diet since mating.

3. When they were reared on the deficient diet, rats bred from mothers on the deficient diet since mating excreted much more methylmalonic acid in the urine than their litter-mates on the supplemented diet. There was wide variation in the level of excretion, both between different animals and from day to day in the same animal.

4. Starvation for more than 16 h caused a marked depression in the amount of methylmalonic acid excreted by rats on the deficient diet.

5. Intraperitoneal injection of sodium propionate into deficient animals after starvation for 24 h caused increased excretion of methylmalonic acid during the following 16 h of continued starvation. Isoleucine had a similar but smaller effect.

6. Tested in the starved animal, sodium propionate and valine given either by intra- peritoneal injection or by mouth, and isoleucine given intraperitoneally, caused increases in the excretion of methylmalonic acid. In contrast, methionine had no effect and threonine only a slight effect.

The absorption of (58Co)cyanocobalamin by unweaned rats

1. ‘58Co’cyanocobalamin was given by stomach tube to Wistar albino rats aged between 6 h and 24 days. The fraction of the dose absorbed was assessed by measuring the amount retained by the animals after 7 days, by total body counting.

2. Rats up to 4 days old absorbed about 90% of the test dose, compared with a mean of 83% at the age of 8 days, 66% at 16 days and 47% at 24 days.

3. The proportion of the test dose absorbed did not appear to be affected by starvation for up to 24 h, by increasing the dose from 0·05 ng to 0·25 ng/g body-weight, or by giving the ‘58Co’cyanocobalamin mixed with rat's milk.

4. The results support earlier suggestions that newborn rats absorb vitamin B12 by a different mechanism from that in adults.

The effect of an atherogenic diet on plasma lipid composition and aortic atherosis in two strains of New Zealand White rabbit

1. Susceptibility to dietary induction of hypercholesterolaemia and aortic atherosis was compared in two groups of male New Zealand White rabbits. 2. Twelve rabbits were purchased from one breeding establishment (group I) and twelve from another (group 2). On arrival at the laboratory six animals from each group were killed and the aortas were removed. Blood samples were taken from the remaining twelve animals and then they were givenad lib.for a period of 40 weeks an atherogenic diet containing 20% butterfat. During this period food intake and body-weight were recorded. At the end of the period blood samples were taken from the animals and immediately afterwards they were killed and the aortas were removed. 3. No atheromatous lesions were found in the aortas of the rabbits in either group killed at the beginning of the experiment. There were no differences between the two groups of animals with respect to body-weight or concentration of cholesterol in the plasma at the beginning of the experiment, food intake during the experiment or body-weight at the end of the experiment. At the end of the experiment, the degree of aortic atherosis in the rabbits of group I was considerably greater than that in the rabbits of group 2. 4. At the end of the experiment the concentrations of total lipids, free cholesterol, esterified cholesterol and phospholipids in the plasma of the rabbits in group 1 were significantly higher than the corresponding concentrations of these lipid components in the plasma of the rabbits in group 2. The concentrations of palmitic, stearic and linoleic acids in the cholesterol esters and the concentration of palmitic acid in the unesterified fatty acids in the plasma of the rabbits in group 2 were significantly higher than the corresponding concentrations of these fatty acids in the plasma cholesterol esters and unesterified fatty acids in the rabbits of group 1. 5. It is concluded that these differences in response to the atherogenic diet were reflections of the differences in the susceptibilities to the dietary induction of hypercholesterolaemia and atherosis of the two differents strain of rabbit that had been established by the two commercial breeders. Such differences in susceptibility could readily explain certain discrepancies in the results of various research workers engaged in this field of investigation.

The effect of diet on the compositions of the triglycerides and unesterified fatty acids isolated from the plasma, liver and adipose tissues of rabbits

1. Groups of male rabbits (ten to thirteen per group survived) were givenad lib. a diet consisting of 80 parts of a low-fat basal diet to which were added: for group I, 20 parts maize oil; for group 2, 20 parts butterfat; for group 3, 10parts maize oil and and 10parts butter- fat; for group 4, 0.47 parts maize oil and 43.1 parts wheat starch; and for group 5, 10.2parts maize oil and 21.6 parts wheat starch. The animals in group 6 were givenan ordinary com- mercial rabbit diet. 2. The rabbits were given the various diets for 38 weeks, after which a sample of blood was taken. The rabbits were then killed and the liver and a sample of perine- phric adipose tissue were removed from each animal. The plasma, liver and adipose tissue lipids werefractionated on columns of Florisil and the fatty acid compositions of the tri- glyceride and unesterified fatty acid fractions were determined by gas-liquid chromatography.

3. The effects of the different diets on the composition of the unesterifiedfatty acids in the plasma were very similar to the effects of the diets on the fatty acid composition of the plasma triglycerides, but in the plasma unesterified fatty acids the concentration of stearic acid was consistently higher and the concentration of linoleic acid was consistently lower than in the plasma triglycerides. 4. There appeared to be a positive rectilinear relationship between the concentration of stearic acid in the plasma triglycerides and the concentration of triglycerides in the plasma. 5. In the triglycerides of the plasma, the concentrations of palmitic and stearic acids were consistently greater and the concentration of linoleic acid was consistently less than the corresponding concentrations of these fatty acids in the triglycerides of the adipose tissues. In the two groups of rabbits given low-fat diets the fatty acid composition of the liver trigly- cerides was almost identical with that of the plasma triglycerides. 6. In each of the six groups of rabbits the composition of the unesterified fatty acids in the plasma was identical with that of the unesterified fatty acids in the adipose tissues. Inthe unesterified fatty acids of the liver the concentrations of linoleic andarachidonic acids were consistently greater and the concentrations of myristic, palmitic and palmitoleic acids were consistently less than the corresponding concentrations of these acids in the unesterified fatty acids of the plasma and adipose tissues. 7. The results of this investigation are discussedin the light of recent con-cepts on the metabolic relationships between the unesterified fatty acids and triglycerides of the liver, plasma and adipose tissues.

THE INFLUENCE OF DIETARY FAT ON THE LIVER LIPIDS OF THE RABBIT

Five groups of rabbits were given diets consisting of 80 parts of a semipurified basal mixture supplemented either with 20 parts of corn oil, 24 parts of butter, 20 parts of butter fat, 20 parts of hydrogenated coconut oil, or with 41.9 parts of starch plus 1 part of corn oil. After 9 months the rabbits were killed and the liver lipids analyzed by silicic acid chromatography. The fatty acids of the liver sterol esters, triglycerides, and phospholipids were analyzed by gas–liquid chromatography. Although there was considerable within-group variation in the concentration of certain of the lipid constituents it was clear that there was no deposition of cholesterol in the liver of the hypocholesterolaemic rabbits given the high corn oil diet. Changes in the fatty acid composition of the three lipid fractions analyzed were confined mainly to increases in the linoleic acid content with the high corn oil diet and to increases in the lauric and myristic acid contents with the hydrogenated coconut oil diet. In the cholesterol ester fraction these increases were balanced by decreases in the oleic acid content but in the triglyceride fraction they were balanced by decreases in the palmitic and palmitoleic acid contents.

THE INFLUENCE OF DIETARY FAT ON THE LIVER LIPIDS OF THE RABBIT

Five groups of rabbits were given diets consisting of 80 parts of a semipurified basal mixture supplemented either with 20 parts of corn oil, 24 parts of butter, 20 parts of butter fat, 20 parts of hydrogenated coconut oil, or with 41.9 parts of starch plus 1 part of corn oil. After 9 months the rabbits were killed and the liver lipids analyzed by silicic acid chromatography. The fatty acids of the liver sterol esters, triglycerides, and phospholipids were analyzed by gas–liquid chromatography. Although there was considerable within-group variation in the concentration of certain of the lipid constituents it was clear that there was no deposition of cholesterol in the liver of the hypocholesterolaemic rabbits given the high corn oil diet. Changes in the fatty acid composition of the three lipid fractions analyzed were confined mainly to increases in the linoleic acid content with the high corn oil diet and to increases in the lauric and myristic acid contents with the hydrogenated coconut oil diet. In the cholesterol ester fraction these increases were balanced by decreases in the oleic acid content but in the triglyceride fraction they were balanced by decreases in the palmitic and palmitoleic acid contents.

SYNTHESIS OF CARBON-14 LABELED DEOXYRIBONUCLEOSIDES

The enzymatic transfer of deoxyribose from a purine or a pyrimidine deoxyriboside to a carbon-14 labeled purine or pyrimidine base affords an efficient means of preparing labeled deoxyribonucleosides. Deoxyadenosine-8-C14, deoxyguanosine-8-C14, deoxycytidine-2-C14, and thymidine-2-C14, with the natural β-configuration, were prepared in good yields and isolated in radiochemically pure form by large-scale paper chromatography.