Catabolism of photo-oxidized and desialylated hemopexin in the rabbit.

Following injection of rabbit 125I-asialohemopexin, more than 90% of the protein-bound 125I was removed from the circulation of rabbits within 12 min. The amount of asialoprotein in the catabolic compartment reached a peak concentration (75 to 85%) 12 min after injection and was completely eliminated from this compartment within 2 hours. The degradation products were excreted into the urine, with 50 to 70% of the 125I eliminated during the first 24 hours and 90 to 95% excreted by 48 hours. Analysis of these data indicated an apparent first order rate constant for uptake of asialohemopexin of 0.32 min-1, for catabolism of 0.020 min-1 and for excretion of 0.054 to 0.093 hour-1. The plasma distribution curves of 125I-hemopexin, after the first 24 hours, showed essentially no difference. Both proteins were catabolized with an average T1/2 of 25 to 26 hours and a similar fractional catabolic rate. Simultaneous injection of heme and 125I-hemopexin resulted in rapid removal and catabolism of the protein. In contrast, injection of heme had little if any effect on the plasma radioactivity curve of photoinactivated 125I-hemopexin.     

Metabolism of 5-thio-D-glucopyranose and 6-thio-D-fructopyranose in rats.

5-Thio-α-d-[U-¹⁴C]glucopyranose and 6-thio-β-d-[U-¹⁴C]fructopyranose were administered orally and intravenously to rats. On intravenous administration of 5-thio-d-[U-¹⁴C]glucopyranose, 1% was oxidized to [¹⁴C]carbon dioxide, 93% was excreted in the urine, and 1.6% was retained in the carcass. On oral administration of 5-thio-d-[U-¹⁴C]glucopyranose, 1% was exhaled as [¹⁴C]carbon dioxide, 90% was excreted in feces and urine, and 4% was retained in the carcass after 72 h. On intravenous administration of 6-thio-β-d-[U-¹⁴C]fructopyranose, 56% was exhaled as [¹⁴C]carbon dioxide, 23% was excreted in the urine, and 7.5% was retained in the carcass; after oral administration, 35% was oxidized to [¹⁴C]carbon dioxide, 50% was excreted in feces and urine, and 6% was retained in the animal after 72 h.On intravenous administration of 5-thio-d-glucose to fasted male rats, blood d-glucose levels increased at lower doses than on oral administration. A dose of 50 mg/kg raised blood d-glucose to 226 mg/100 ml within 2.5 h after intravenous but only to 173 mg/100 ml within 2 h after oral administration from basal level of 70–90 mg/100 ml. Blood d-glucose concentration returned to normal levels within 9 h in both cases. 6-Thio-d-fructopyranose showed no diabetogenic action. The LD₅₀ of 6-thio-d-fructopyranose was 11,200 mg/kg when tested in mice.     

Energy metabolism of beating rat heart cell cultures: II. - Glucose metabolism

Glycogen metabolism, lactate excretion and carbon dioxide production from uniformly labelled glucose were studied on beating rat heart cell cultures in relation to medium replenishment. Only a small fraction of labelled glucose was incorporated into glycogen and then released at a high rate, while exogenous glucose was still continuously taken up by the cells. When glucose in the medium was nearly depleted, glycogen degradation slowed down. The rate of carbon flow through phosphopentose shunt showed a 600 p. cent increase several hours before DNA synthesis. Up to 24 hours after medium renewal, the cells excreted high amounts of lactate; then lactate was reutilized, probably through transaminating processes. Up to 10 hours after medium renewal, labelled carbon dioxide production from exogenous labelled glucose decreased strongly and then increases. Only one of these variations (pentose phosphate shunt) could be attributed to the release of topoinhibition by serum. The other three also occured in the absence of serum and probably resulted from the variation in substrate avaibility induced by medium replenishment.     

Metabolism of intravenously administered 7 alpha-hydroxycholesterol-3 beta-stearate in the hamster.

In order to investigate the metabolic fate of serum esterified 7 alpha-hydroxycholesterol, [4-14C]7 alpha-hydroxycholesterol-3 beta-stearate was synthesized from labeled cholesterol and administered to bile fistula hamsters intravenously. Bile samples were collected at every 20 min for 7 h. Radioactivity was detected in bile 40 min after the beginning of the infusion of the labeled compound and 56.5 +/- 5.7% (48.7-66.0%) of the administered radioactivity was recovered in bile during 7 h. The liver contained appreciable radioactivity (19.5 +/- 7.6% of the administered dose) at the time of sacrifice. Only a trace amount of radioactivity was detected in urine and blood. Cumulative recovery of the radioactivity was 76.3 +/- 8.6% (63.3-90.4%). Major radioactive metabolites in the bile samples were identified to be taurine- and glycine-conjugated cholic acid and chenodeoxycholic acid by radioactive thin-layer chromatographic analysis of the bile samples before and after enzymatic hydrolysis and 3 alpha-hydroxysteroid dehydrogenase treatment. The conversion was nearly complete and we could not detect neutral metabolites, such as the mother compound, free 7 alpha-hydroxycholesterol and bile alcohols, as well as glucuronidated or sulfated bile acids. It is concluded that serum esterified 7 alpha-hydroxycholesterol could be effectively taken up by the liver, hydrolyzed by cholesterol esterase and metabolized via the normal biosynthetic pathway to taurine- or glycine-conjugated primary bile acids to be excreted into bile.     

(14C)Ethylenethiourea: distribution, excretion, and metabolism in pregnant rats.

Following administration of single oral doses of [14C]ethylenethiourea (ETU) to pregnant rats maternal blood maintained peak radioactivity for 2 h, and the radioactivity was dispersed uniformly between the red blood cells and plasma. The level of radioactivity was distributed equally among several maternal tissues but was present in lower amounts in embryos. Twenty-four hours after treatment all tissues examined, except blood, were relatively clear of radioactivity and 72.8% of the total radioactivity given had been excreted in the urine. Elution patterns of metabolites from Sephadex separation suggested that ethylenethiourea was degraded very little. The teratological mechanism is discussed.     

The excretion and degradation of chondroitin 4-sulphate administered to guinea pigs as free chondroitin sulphate and as proteoglycan

The excretion and degradation was studied of (35)S-labelled 4-chondroitin sulphate injected into guinea pigs in the form of proteoglycan isolated from cartilage and in the form of free chondroitin 4-sulphate prepared from the same proteoglycan by proteolysis. When the proteoglycan was injected there was a delay of about 15-20min before significant amounts or radioactivity were excreted, whereas after injection of chondroitin 4-sulphate a considerable amount of radioactivity was excreted within 10min and a much higher proportion of the radioactive dose was excreted in 1h or 24h compared with the proteoglycan. In both cases, however, a major part of the radioactivity was not excreted even in 24h. Sterile conditions were used to collect the radioactive material directly from the bladder. When chondroitin 4-sulphate was injected, the molecular sizes of injected and excreted materials were similar, as assessed by gel chromatography on Sephadex G-200, whereas when proteoglycan was injected the molecular size of the excreted labelled material was similar to that of the chondroitin 4-sulphate chains in the original proteoglycan. In neither case did the size of the excreted labelled material change with time over 1h, and low-molecular-weight labelled material was virtually absent. In contrast, when urine was collected for 24h without preservative the labelled material in it was extensively degraded after either the proteoglycan or chondroitin 4-sulphate had been given. Chondroitin 4-sulphate became similarly degraded when incubated with non-sterile urine, but not when the urine was passed through a bacterial filter, suggesting that degradation was caused by contaminating micro-organisms in the experiments in which urine was collected for 24 h. It is concluded that chondroitin 4-sulphate chains of about 18000 molecular weight can be excreted readily as such, whereas intact proteoglycans must be degraded to free glycosaminoglycans first, although both are taken up by the tissues more rapidly than they are excreted.     

Metabolic Fate of Sarcosyl-14C-glycine in Normal Young Rats

The metabolic fate of the non-physiological synthetic dipeptide, sarcosyl-[2-¹⁴C]glycine, was investigated in normal young rats in vivo and in vitro compared to that of seryl-[2-¹⁴C]glycine as a typical example of common physiological dipeptides. The radioactive dipeptides were synthesized from sarcosine or L-serine and [2-¹⁴C]glycine in our laboratory. When radioactive sarcosylglycine was given intraperitoneally, about 30% of the dose was excreted in the urine, and more than 90% of the urinary radioactivity was present in the sarcosylglycine fraction. The recovery of radioactivity in the the expired carbon dioxide and body protein was 8 and 50% of the dose, respectively, during a 24-h period. When the labeled serylglycine was given, the recovery of radioactivity in the urine was 8% of the dose, and 15% in the expired carbon dioxide. In slices of the kidney, liver and small intestine from normal rats, serylglycine was rapidly and almost completely hydrolyzed, and a large amount of free glycine was released. However, sarcosylglycine was hardly hydrolyzed to the corresponding amino acids in the liver and small intestine, and only slightly in the kidney. These results suggest that a considerable amount of sarcosylglycine given intraperitoneally was rapidly excreted into the urine of normal young rats, reflecting less sensitivity to hydrolysis by tissue peptidase(s) when compared to serylglycine.     

Orthophosphate excretion as related to RNA metabolism in Tetrahymena

Log phase cultures of Tetrahymena pyriformis W excrete orthophosphate, a purine, and a pyrimidine when suspended in a non-nutrient buffered medium. Ribonucleic acid has been established as the primary source of these catabolic products. Thirty per cent of the total cellular RNA was degraded in three hours under the conditions of growth and suspension employed. The size of the phosphorus pools of these ciliates was determined during the period of RNA degradation; the internal acid-soluble organic phosphate and the cellular orthophosphate pools remained constant for five hours at 20% and 5% of the total cell phosphorus respectively. The ultraviolet-absorbing materials excreted with the orthophosphate were identified as hypoxanthine and uracil. A pentose, presumably ribose, was excreted in small quantities, but was not equivalent in amount to the orthophosphate and did not parallel the pattern of release of the anion. The excretion of orthophosphate, hypoxanthine, and uracil is correlated with the catabolism of RNA as it has been demonstrated in these cells.     

Colorimetric analysis with N,N-dimethyl-p-phenylenediamine of the uptake of intravenously injected horseradish peroxidase by various tissues of the rat

1. A method is described for the colorimetric determination of peroxidase with N,N-dimethyl-p-phenylenediamine. The amount of red pigment formed by peroxidase is proportional to the concentration of enzyme and to the time of incubation during the first 40 to 90 seconds. The influence of the concentration of enzyme, N,N-dimethyl-p-phenylenediamine, H(2)O(2), the time of incubation, pH, the temperature, and the possible interference by oxidizing and reducing agents of tissues has been tested. 2. The method has been used to follow the uptake of intravenously injected horseradish peroxidase by 18 different tissues of the rat over a period of 30 hours. The highest concentration of the injected tracer enzyme was found in extracts of kidney, liver, bone marrow, thymus, and spleen. Considerable amounts were taken up by pancreas, prostate, epididymis, and small intestine. Lower concentrations were found in extracts of lung, stomach, heart, and skeletal muscle, aorta, skin, and connective tissue. No uptake was observed by brain and peripheral nerve tissue. 3. Tissue homogenates containing high concentrations of the injected peroxidase, in general also showed high or average activity of acid phosphatase. 4. Six hours after intravenous administration, the liver contained 27 per cent, the kidney 12 per cent, and the spleen, 1.4 per cent of the injected dose. 5. Approximately 20 per cent of the injected peroxidase was excreted in the urine during the first 6 hours, and the concentration of peroxidase in blood serum and urine fell exponentially during this time. After 6 hours, only low concentrations were excreted in the urine but low enzyme activity was still detectable after 30 hours. Approximately 6 per cent of the injected dose was excreted in the feces from 6 to 20 hours after administration. 6. After feeding through a stomach tube, low concentrations of peroxidase were found in blood serum and urine. Considerable variations in the extent of absorption from the gastrointestinal tract were observed in individual rats.     

Reduced renal reabsorption of 1,5-anhydro-D-glucitol in diabetic rats and mice

A stable amount, approximately 60 micrograms, of 1,5-anhydro-D-glucitol (AG) was detected in the 24-h-urine of normal young rats fed ad libitum. Upon administration of streptozotocin (STZ), this amount was temporarily elevated to as much as 1.1 mg and AG was concomitantly removed from the circulation. The plasma AG level stayed almost null thereafter while the acutely elevated urinary AG excretion declined within 24 h to another stable excretion level that was three times as high as that of the untreated rats. In contrast, glucosuria developed much more slowly in the drug-treated rats. Normal rats and mice retained exogenous [14C]AG to a considerable extent and the radioactivity was distributed all over the body. Only a marginal fraction of the radioactivity was excreted as expired CO2. The radio-activities retained in the body and excreted into the urine were mostly attributed to unmetabolized AG. The observations of AG's metabolic stability and its relatively low level of leakage into urine suggested the concept of effective renal AG reabsorption. On the other hand, the rats with STZ-induced diabetes and NOD-mice with spontaneously developed diabetes retained little of the radioactive AG in their bodies; most of the injected radioactivity was recovered in the urine within 24 h. This observation was interpreted as due to reduced renal AG reabsorption in these animals. The concept of reduction in renal AG reabsorption in diabetes could account for the reduced plasma AG level generally observed in human diabetic cases.     

The biotransformation and urinary excretion of dexamethasone in equine male castrates

The pro-drugs of dexamethasone, a potent glucocorticoid, are frequently used as anti-inflammatory steroids in equine veterinary practice. In the present study the biotransformation and urinary excretion of tritium labelled dexamethasone were investigated in cross-bred castrated male horses after therapeutic doses. Between 40-50% of the administered radioactivity was excreted in the urine within 24 h; a further 10% being excreted over the next 3 days. The urinary radioactivity was largely excreted in the unconjugated steroid fraction. In the first 24 h urine sample, 26-36% of the total dose was recovered in the unconjugated fraction, 8-13% in the conjugated fraction and about 5% was unextractable from the urine. The metabolites identified by microchemical transformations and thin-layer chromatography were unchanged dexamethasone, 17-oxodexamethasone, 11-dehydrodexamethasone, 20-dihydrodexamethasone, 6-hydroxydexamethasone and 6-hydroxy-17-oxodexamethasone together accounting for approx 60% of the urinary activity. About 25% of the urinary radioactivity associated with polar metabolites still remains unidentified.     

COLORIMETRIC INVESTIGATION OF THE UPTAKE OF AN INTRAVENOUSLY INJECTED PROTEIN (HORSERADISH PEROXIDASE) BY RAT KIDNEY AND EFFECTS OF COMPETITION BY EGG WHITE

After intravenous injection of horseradish peroxidase into rats, the foreign protein appeared in the kidney first in the small phagosomes and its concentration there decreased quickly; it then was concentrated and "stored" for several days in the large phagosomes. After injection of 10 mg of peroxidase per 100 gm of body weight, the concentration of peroxidase in blood and urine decreased exponentially during the first 6 hours; small amounts of peroxidase were excreted in the urine for several days. When 0.05 to 1.0 mg of peroxidase per 100 gm were administered, most of the peroxidase was taken up by the liver and little by the kidney, and a portion was excreted in the urine even at the lowest dose. At doses above 1.5 mg per 100 gm, the liver cells were saturated, and large reabsorption droplets appeared in the tubule cells of the kidney. With further dosage increase, the concentration of peroxidase in the phagosomes of the kidney increased rapidly until saturation was reached at doses of 13 mg per 100 gm. After intraperitoneal injection of egg white 18 hours prior to the administration of peroxidase, the concentration of peroxidase in all kidney fractions was only 10 to 25 per cent of the values for the untreated animals, the disappearance of peroxidase from the blood was delayed, and 81 percent more peroxidase was excreted in the urine. The treatment with egg white had no effect on the uptake of peroxidase by the liver. The ability of kidney tissue to degrade and adsorb peroxidase in vitro was tested.     

Absorption and excretion of tablet disintegrating β-cyclodextrin polymer in rats

Absorption and excretion of a new tablet disintegrating agent, a crosslinked β-cyclodextrin polymer was investigated following per os administration in the rat. The polymer, which is insoluble but swells in water, was prepared from β-cyclodextrin by reacting with [2-¹⁴C]epichlorohydrin in an alkaline medium. Radioactivity of blood, urine, faeces, exhaled carbon dioxide and the gastrointestinal tract was determined by a liquid scintillation method. No radioactivity could be detected in the blood up to 24 h after the administration of the polymer. Radioactivity of urine and exhaled carbon dioxide together did not exceed 0·11% of the total administered radioactivity, 98% of which was found in the large intestine and the faeces. Therefore, it is assumed that β-cyclodextrin polymer could not be absorbed from the gastrointestinal tract.     

Kinetics of phenazepam-14C excretion from the body of white rats in the single and prolonged administration of the preparation]

During 5 days after intraperitoneal injection of 14C-phenazepam into albino rats, about 77% of the total radioactivity was excreted with urine and feces in both intact animals and in those premedicated with phenazepam for 15 days. The excretory processes are described by the first order equations. The rates of phenazepam total excretion are identical in single and repeated injections. At the same time, phenazepam injected into the animals at a single dose is predominantly excreted with urine, while in multiple administration it is excreted with feces. Excretion of phenazepam with urine acquires the biexponential features, provided it is injected in multiple doses.     

In vivo metabolism of leukotriene C4 in man: urinary excretion of leukotriene E4

Five - 20 nmoles of [5,6,8,9,11,12,14,15-3H8]leukotriene C4 was injected into three male volunteers. Forty-eight percent of the administered 3H was recovered from urine and 8% from feces, within a 72 hr period. Of the total urinary radioactivity 44% was excreted during the first hour after injection. This activity was mainly found in one compound, designated "I". The radioactivity excreted into urine later than one hour after injection, consisted partly of Compound I and two additional components, and partly of polar, non-volatile material. Compound I was identified as leukotriene E4 by UV-spectroscopy and cochromatographies in three high performance liquid chromatography systems with synthetic reference compounds. A total of 13% of administered radioactivity was excreted in urine as leukotriene E4.     

Evaluation of urinary dihydrocodeine excretion in human by gas chromatography–mass spectrometry

Urinary metabolic pattern after the therapeutic peroral dose of dihydrocodeine tartrate to six human volunteers has been explored. Using the GC–MS analytical method, we have found that the major part of the dose administered is eliminated via urine within the first 24 h. However, the analytical monitoring of dihydrocodeine and its metabolites in urine was still possible 72 h after the dose was administered. The dihydrocodeine equivalent amounts excreted in urine in 72 h ranged between 32 and 108% of the dose, on average 62% in all individuals. The major metabolite excreted into urine was a 6-conjugate of dihydrocodeine, then in a lesser amount a 6-conjugate of nordihydrocodeine (both conjugated to approximately 65%). The O-demethylated metabolite dihydromorphine was of a minor amount and was 3,6-conjugated in 85%. Traces of nordihydromorphine and hydrocodone were confirmed as other metabolites of dihydrocodeine in our study. This information can be useful in interpretation of toxicological findings in forensic practice.     

Colorimetric Analysis with N,N-Dimethyl-p-phenylenediamine of the Uptake of Intravenously Injected Horseradish Peroxidase by Various Tissues of the Rat

1. A method is described for the colorimetric determination of peroxidase with N,N-dimethyl-p-phenylenediamine. The amount of red pigment formed by peroxidase is proportional to the concentration of enzyme and to the time of incubation during the first 40 to 90 seconds. The influence of the concentration of enzyme, N,N-dimethyl-p-phenylenediamine, H₂O₂, the time of incubation, pH, the temperature, and the possible interference by oxidizing and reducing agents of tissues has been tested. 2. The method has been used to follow the uptake of intravenously injected horseradish peroxidase by 18 different tissues of the rat over a period of 30 hours. The highest concentration of the injected tracer enzyme was found in extracts of kidney, liver, bone marrow, thymus, and spleen. Considerable amounts were taken up by pancreas, prostate, epididymis, and small intestine. Lower concentrations were found in extracts of lung, stomach, heart, and skeletal muscle, aorta, skin, and connective tissue. No uptake was observed by brain and peripheral nerve tissue. 3. Tissue homogenates containing high concentrations of the injected peroxidase, in general also showed high or average activity of acid phosphatase. 4. Six hours after intravenous administration, the liver contained 27 per cent, the kidney 12 per cent, and the spleen, 1.4 per cent of the injected dose. 5. Approximately 20 per cent of the injected peroxidase was excreted in the urine during the first 6 hours, and the concentration of peroxidase in blood serum and urine fell exponentially during this time. After 6 hours, only low concentrations were excreted in the urine but low enzyme activity was still detectable after 30 hours. Approximately 6 per cent of the injected dose was excreted in the feces from 6 to 20 hours after administration. 6. After feeding through a stomach tube, low concentrations of peroxidase were found in blood serum and urine. Considerable variations in the extent of absorption from the gastrointestinal tract were observed in individual rats.     

Distribution of isotopic label after the oral administration of free and bound 14C-labelled glucosamine in rats

The metabolic fate of [1-(14)C]glucosamine, of N-acetyl[1-(14)C]glucosamine and of glycoproteins labelled with [1-(14)C]glucosamine was studied in rats for a period of 24hr. after these materials were given orally or injected. When [1-(14)C]glucosamine was injected 26.3% of the label was excreted in the urine, 19.7% was expired as carbon dioxide and 12.7% was incorporated into plasma proteins. When the same compound was given orally, 49.2% of the label was expired as carbon dioxide, with little appearing in the urine or in the plasma. When N-acetyl[1-(14)C]glucosamine was injected, 51.3% of the label was excreted in the urine with 12.3% appearing in carbon dioxide, but there was little incorporation into plasma protein. When this compound was given orally, 46.5% of the label was expired as carbon dioxide, 7.4% was recovered in the urine and 1.7% was incorporated into plasma protein. After the injection of (14)C-labelled glycoprotein 21.0% of the label was expired as carbon dioxide, whereas when it was given orally 49.8% of the label was recovered in carbon dioxide. The differences observed between the metabolic fate of the amino sugars when they were given orally and their fate when injected could not be accounted for by the action of the intestinal microflora or by the rate of administration of the material. It is concluded that amino sugars undergo metabolic alteration or degradation during absorption.     

The metabolism of estriol in rabbits

Rabbits have been shown to excrete 6, 7-³H-estriol, its conjugates and metabolites preponderantly in the bile during the initial 4 hours following the I.V. injection of the labeled steroid. The amount of radioactivity excreted in the urine was $\text{1}\text{3}$ of that in the bile. Since in intact rabbits most of the injected radioactivity of ³H-estriol is excreted in the urine over a period of days (and very little in the feces), it appears that estriol and its conjugates and metabolites are involved in an efficient enterohepatic. circulation. In the bile, the preponderant metabolite of ³H-estriol was the 3-glucosiduronate. Even though the latter constituted a substantial part of the urinary metabolites, other conjugates and metabolites of estriol were present in considerable amounts. It is possible that the latter have resulted from gastro-intestinal and/or renal metabolism. Incubation of rabbit liver with estriol led to 75% conjugation with glucuronic acid in the 3-position.     

Measurement of bile acid synthesis by 14CO2: the metabolism of propionyl CoA.

The relationship between 14CO2 evolution from the catabolism of [26 or 2714C] cholesterol to bile acids was studied in rats with biliary fistulae. When equal quantities of [26 or 2714C] cholesterol and [414C] cholesterol were administered, there was a significant linear relationship between 14CO2 expiration in the breath and [414C] bile acid excreted in the bile. Bile acid synthesis calculated as the ratio of 14CO2: molar specific activity of biliary cholesterol correlated highly with biliary bile acid excretion in the bile acid depleted rat. Phenobarbital, a known inducer of gamma-amino levulenic acid formation from succinyl CoA did not alter the relationship between the 14CO2 estimation of bile acid synthesis and biliary bile acid excretion, indicating that the relationship between [26 or 2714C] cholesterol side chain cleavage and 14CO2 formation was not altered. Phenobarbital, however, did cause a reduction in bile acid synthesis measured by 14CO2 evolution and by biliary bile acid excretion. The 14CO2 method underestimated bile acid excretion. 8.7% in untreated and phenobarbital treated rats respectively. Since 11% of the radioactivity which was expired as 14CO2 was isolated as bile acids, radioactivity cleaved as [1 or 314C] propionyl CoA may enter cholesterol-bile acid biosynthesis resulting in the underestimation of bile acid synthesis. To test whether radioactivity from propionyl CoA enters steroid biosynthesis [114C] propionate and [214C] propionate were given to untreated biliary fistula rats and the biliary lipids excreted in 60 hours were analyzed. Incorporation of radioactivity into cholesterol and bile acids was greater after the administration of [214C] propionate than after [114C] propionate than after [114C] propionate, suggesting that radioactivity from propionyl CoA may enter steroid biosynthesis by metabolic events in which the methylene and carboxyl carbon atoms are differentiated. Although the use of 14CO2 expiration from [26 or 2714C] cholesterol catabolism underestimates the rate of bile acid synthesis, it should have many applications because of the constant relationship between 14CO2 formation and cholesterol side chain cleavage.