Clearing-factor lipase in adipose tissue. Distinction of different states of the enzyme and the possible role of the fat cell in the maintenance of tissue activity

1. Incubation of intact epididymal adipose tissue from fed rats at 37 degrees in an albumin solution at pH7.4 in vitro results in rapid loss of clearing-factor lipase activity until a low activity, stable to prolonged incubation, is attained. The clearing-factor lipase activity of intact tissue from starved rats, which is initially much less than that of tissue from fed rats, is mainly stable to incubation at 37 degrees . 2. Much of the clearing-factor lipase activity of intact epididymal adipose tissue from fed rats is inactivated by collagenase. The enzyme activity of intact tissue from starved rats is not inactivated by collagenase. 3. The clearing-factor lipase activity of fat cells isolated from the epididymal adipose tissue of fed rats is stable to prolonged incubation at 37 degrees . It represents only a small proportion of the total activity of the intact tissue. In starved rats, the isolated fat cells contain a much higher proportion of the activity of the intact tissue. Their activity is also stable at 37 degrees . 4. Incubation of isolated fat cells in a serum-based medium leads to a progressive rise in clearing-factor lipase activity. Actinomycin increases the extent of this rise in activity. No rise in clearing-factor lipase activity occurs when stromal-vascular cells isolated from epididymal adipose tissue are incubated in the medium. 5. The findings indicate that less than 20% of the activity of intact adipose tissue from fed rats is retained when fat cells are isolated from the tissue by collagenase treatment. The activity that is lost could be that which normally functions in the uptake of triglyceride fatty acids by the tissue.

Clearing-factor lipase in adipose tissue. A possible role of adenosine 3',5'-(cyclic)-monophosphate in the regulation of its activity

1. The rise in clearing-factor lipase activity that occurs when epididymal fat bodies from starved rats are incubated in appropriate media in vitro is inhibited in the presence of 6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP (1mm). 2. Inhibition occurs at a concentration of glucose in the incubation medium of 1.3mg./ml. or less, but not at a glucose concentration of 2.4mg./ml., unless caffeine (1mm), an inhibitor of 3',5'-(cyclic)-nucleotide phosphodiesterase, is also present. Caffeine (5mm) alone inhibits the rise in clearing-factor lipase activity at a glucose concentration of 2.4mg./ml. of medium. 3. The concentration of free fatty acids in the epididymal fat bodies normally falls during incubations in vitro as the rise in clearing-factor lipase activity occurs. In the presence of 1mm-6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP, however, either the tissue free fatty acid concentration is increased or it does not fall to the same extent. The concentration of glucose in the incubation medium is important in determining the direction and extent of the changes in tissue free fatty acid concentration that occur in the presence of 6-N-2'-O-dibutyryl-3',5'-(cyclic)-AMP. 4. Free fatty acid concentrations in epididymal fat bodies in vivo rise as the clearing-factor lipase activity of the tissue falls during starvation. 5. The possibility that the concentration of 3',5'-(cyclic)-AMP in adipose tissue may regulate clearing-factor lipase activity, and that the regulation may occur through effects of the nucleotide on tissue free fatty acid concentrations, is discussed.

Clearing-factor lipase in adipose tissue. Studies with puromycin and actinomycin

1. When adipose tissue from starved rats is incubated in a medium containing glucose, insulin, heparin and actinomycin (5mug./ml.) the total clearing-factor lipase activity of the system increases at least tenfold over a period of 9hr. In the absence of actinomycin, enzyme activity also increases, but to a lesser extent and for only about 3hr. Some enzyme activity appears in the incubation medium in both the presence and the absence of actinomycin. 2. When the glucose and insulin of the incubation medium are replaced by pyruvate and heparin is omitted, an increase in the total clearing-factor lipase activity in the presence of actinomycin still occurs, but only after a lag of several hours. When only heparin is omitted from the medium, the rise in enzyme activity begins immediately, but there is a shoulder in the time-course curve after a few hours. In the absence of heparin, little enzyme activity appears in the incubation medium. 3. The increases in enzyme activity in the presence of actinomycin are prevented if puromycin (0.5mg./ml.) is present in the incubation medium. 4. Catecholamines and corticotrophin inhibit the increase in enzyme activity caused by actinomycin. 5. The clearing-factor lipase activity of adipose tissue from fed animals declines with a half-life of between 1 and 1.5hr. when the tissue is incubated in the presence of puromycin. The clearing-factor lipase activity of adipose tissue from starved animals is stable under similar circumstances, as is the raised activity found after such tissue has been incubated in the presence of actinomycin. 6. Clearing-factor lipase extracted from adipose tissue of fed animals is less stable in solution than that extracted from the tissue of starved animals after this has been incubated in the presence of actinomycin.

The significance of changes in tissue clearing-factor lipase activity in relation to the lipaemia of pregnancy

1. The concentration of triglyceride fatty acid in the plasma of the pregnant rat rises to a maximum 2-4 days before parturition. Thereafter there is a rapid decline in the concentration to near normal values at parturition. 2. A similar increase occurs in animals fed on a diet low in fat. There is no increase in food consumption at the time when the triglyceride fatty acid concentration in the plasma is at its peak. 3. Rates of entry of triglyceride fatty acid into the blood during pregnancy have been estimated from the rate of accumulation of triglyceride in the plasma of animals injected with a non-ionic detergent, Triton. A progressive increase occurs in the entry rate as the body weight increases throughout pregnancy. Expressed per constant body weight, the entry rate does not change significantly. 4. Adipose-tissue clearing-factor lipase activity is low at the time when the plasma triglyceride fatty acid concentration is raised. Activity of the enzyme in heart, lung and diaphragm is unchanged. 5. It is suggested that the ;lipaemia of pregnancy' may be due to diminished uptake of triglyceride fatty acids by adipose tissue, and, further, that the disappearance of the lipaemia may be due to increased uptake of triglyceride fatty acids by the mammary gland.

Metabolism of triglyceride fatty acid by the perfused rat heart

1. Chyle lipids, labelled with (14)C, are taken up and oxidized by the isolated perfused rat heart. 2. In recirculatory perfusions, when chyle lipids are the sole exogenous energy source, about 24% of the total oxygen uptake is accounted for by their oxidation. This proportion is not changed by starvation of the rats for 48hr. and falls when an external work load is imposed on the left ventricle. 3. With albumin in the perfusion medium, the rate of (14)CO(2) output is reduced by half and there is a rise in the proportion of (14)C-labelled free fatty acids in the medium. 4. Clearing-factor lipase appears in the perfusion medium when chyle lipids are perfused through the heart. In the absence of albumin, the activity of the medium enzyme is low and only a small proportion of the (14)CO(2) output can be accounted for by the oxidation of free fatty acids released by it. In the presence of albumin, the enzyme is more active in the medium. 5. When a substantial proportion of the total clearing-factor lipase is removed from the heart by a prior perfusion with heparin, (14)C-labelled chyle lipid perfused subsequently is oxidized at only half the normal rate.

The use of a non-ionic detergent (Triton WR 1339) to determine rates of triglyceride entry into the circulation of the rat under different physiological conditions

1. The concentration of triglyceride in the plasma rises linearly for 3 hr after the intravenous injection of the non-ionic detergent, Triton WR 1339, in the rat.2. Minimum rates of triglyceride entry into the plasma have been calculated from measurements of the plasma triglyceride concentration at 1.5 hr after Triton injection. These are 30 and 52 mumoles/hr.200 g body weight respectively for male and female animals on a stock laboratory diet and 25 and 33 mumoles/hr.200 g body weight respectively for male and female animals starved for 48 hr. The values may have to be increased by 10 or 20% to take account of a small leak of triglyceride from the plasma in animals injected with Triton and of an effect of ether anaesthesia on the rate of entry of triglyceride into the plasma.3. Substitution of the stock laboratory diet by diets that are lower in fat content does not alter significantly the rate of triglyceride accumulation in the plasma after Triton injection.4. There is no change in the triglyceride content of the livers of male and female rats at 24 hr after Triton injection. At 6 hr after Triton, the liver triglyceride content of female rats which have been fed their normal diet is increased.

The effect of a non-ionic detergent (Triton WR 1339) on the removal of triglyceride fatty acids from the blood of the rat

1. The initial rate of removal from the circulation of injected [(14)C] labelled chylomicron triglyceride is much slower in rats that have been injected with the non-ionic detergent, Triton WR 1339, than in normal animals. Pre-incubation of the lipid with Triton before its injection causes a further reduction in removal rate.2. After the injection of [(14)C]labelled chyle triglyceride into animals given Triton, the rate of disappearance of radioactivity falls markedly as the percentage of the labelled material remaining in the blood declines.3. A much smaller decrease in the rate of removal of injected chylomicron triglyceride occurs with increase in the size of the injected dose and with increase in the time interval between Triton and chylomicron triglyceride injections.4. Serum triglycerides labelled with (14)C are removed from the circulation of rats that have been given Triton at a much slower rate than [(14)C] labelled chylomicron triglycerides.

Clearing-factor lipase in adipose tissue. Factors influencing the increase in enzyme activity produced on incubation of tissue from starved rats in vitro

1. Evidence is presented that the increase in clearing-factor lipase activity that occurs when adipose tissue from starved rats is incubated in a defined medium in vitro is due to an increase in the total enzyme content of the system. It is shown that the clearing-factor lipase activity rises to reach a plateau level where, it is suggested, rates of enzyme synthesis and of enzyme destruction become balanced. 2. The presence of heparin in the incubation medium results in the extraction of part of the clearing-factor lipase originally present in the adipose tissue and this could provide the stimulus for the increase in total enzyme content. 3. Glucose is required in the incubation medium at a very low concentration. It can be replaced by fructose, but not by pyruvic acid, lactic acid, glyceric acid or dihydroxyacetone. 4. Adrenaline and corticotrophin inhibit the increase in enzyme activity when they are present in the incubation medium. 5. The high clearing-factor lipase activity associated with adipose tissue of fed rats is decreased by 50% within 3hr. of the injection of puromycin.

Clearing-factor lipase in adipose tissue. A medium in which the enzyme activity of tissue from starved rats increases in vitro

1. When epididymal fat bodies from starved rats are incubated for 3.5hr. at 37 degrees in a defined medium in vitro the total clearing-factor lipase activity rises to approximately twice its initial value. 2. During the incubation period part of the tissue clearing-factor lipase activity appears in the medium. 3. Heparin, glucose, insulin, and HCO(3) (-) and K(+) ions are shown to be important medium constituents.