Himanshu V. Kothari

Aortic cholesterol esterase: Characteristics of normal rat and rabbit enzyme

Abstract The enzymes catalyzing the synthesis and hydrolysis of cholesteryl esters were studied in the acetone dry powder of normal aorta of male rat and rabbit. In both species the enzymes were found to have similar stability, solubility and kinetic properties. The synthetic enzyme was found to be most effective with an emulsified substrate and had a pH maximum of 6.2. Exogeneous ATP or CoA did not stimulate the activity. Sodium taurocholate was essential for activity but was found inhibitory at higher concentrations. The hydrolytic enzyme was about three times more effective when using a micellar substrate than for the emulsified substrate. The enzyme preparation exhibited two pH optima for hydrolysis, depending upon the physical state of the substrate. The micellar substrate was optimally hydrolyzed at pH 6.6 and the albumin dispersed substrate at pH 7.4. In the rat aorta preparation, the specific activities for hydrolysis and synthesis were 9.5 and 5.7, respectively; in the rabbit they were 3.0 and 3.3, respectively.

Increased activity of lysosomal enzymes in human atherosclerotic aortas

Abstract Beta-glucuronidase activity is known to increase in human atherosclerotic arteries. Since beta-glucuronidase is a key marker enzyme for lysosomal activity, it was decided to investigate the activity of other lysosomal enzymes. In intima-media homogenates of human atherosclerotic aortas there was found an increased total activity of cathepsin, suggestive evidence for acid phosphatase, and no change in arylsulfatase. Beta-glucuronidase showed the greatest increase in total activity. Besides showing these changes in total activity, all four enzymes exhibited definitely increased activity in the lysosomal fraction of the atherosclerotic aortas.

Cholesterol ester hydrolase in homogenates and lysosomal fractions of human aorta

Abstract Evidence has been obtained for the presence of cholesterol ester hydrolase in the normal human aorta. In 22 aortas the total activity was 168 units per g of tissue with 16% contained in the lysosomal fraction. The enzyme preparation exhibits two pH optima depending upon the physical state of the substrate. Micellar stabilized substrate is optimally hydrolyzed at pH 6.6 and albumin dispersed substrate at a pH optimum of 7.4. The results of sub-cellular fractionations suggest that the enzyme acting on micellar substrate is lysosomal in nature. Data are presented on the effects of ions and inhibitors and the requirement of bile salt in the enzymatic reaction.

Aortic cholesterol esterase: studies in White Carneau and Show Racer pigeons.

Abstract The aortic cholesterol esterase of pigeons of the Show Racer and White Carneau strains was studied using acetone dry powder preparations. The White Carneau pigeon is susceptible to atherosclerosis and its ratio of aortic cholesterol ester synthesizing to hydrolyzing activity was 1.21 compared to 0.73 for Show Racer pigeons. The synthetase/ hydrolase ratio for proximal and distal portions of the aorta was 0.48 and 0.52 for Show Racer pigeons and 0.65 and 0.95 for White Carneau pigeons.

Arterial enzymes of cholesteryl ester metabolism.

Publisher Summary This chapter discusses arterial enzymes of cholesteryl ester metabolism. The atherosclerotic plaque is characterized by lipid accumulation and a variable connective-tissue reaction. The important feature of atheroma lipids is the predominance of cholesterol, particularly in its esterified form. The arterial wall can synthesize the principal types of lipids. Cholesterol feeding stimulates phospholipid synthesis in the arteries of experimental animals. Cholesterol esterases catalyze the hydrolysis of cholesterol esters, and also the synthesis of these esters from cholesterol and free fatty acids. A characteristic feature of cholesterol ester hydrolase as a synthesizing enzyme is its independence from energy donors, in particular from adenosine triphosphate. The stimulation of cholesterol esterification is one of the earliest changes in atherogenesis. Susceptibility to atherosclerosis varies widely among different species, and susceptibility varies from one anatomical site to another while within a given species.

Lysosomal enzymes in aortas of species susceptible and resistant to atherosclerosis.

Summary Lysosomal enzyme activity in grossly normal aortas of species susceptible and resistant to atherosclerosis has been studied. β-Glucuronidase, cathepsin, acid phosphatase, arylsulfatase B, and cholesteryl ester hydrolase were the lysosomal-like enzymes measured. Resistant species, the rat, guinea pig and dog, showed higher total lysosomal enzyme activity than did the susceptible species, rabbit and swine. Using malate dehydrogenase as an indicator, no difference in mitochondrial enzyme activity was observed in the various species.

Aortic cholesterol esterase in rabbits: Effect of duration of cholesterol feeding*

Summary The cholesteryl ester synthesizing (S) and hydrolysing (H) activities of rabbit aorta have been assayed in rabbits fed an atherogenic regimen (2% cholesterol in 6% corn oil). Both synthesis and hydrolysis of cholesteryl esters increase over control levels, but the increase in synthetase activity is much greater than that of hydrolase activity. The S/H ratio is sharply increased after 5–7 days on the atherogenic diet, a time when serum and liver cholesterol levels have risen but no atheromata are grossly visible.

Increased activity of lysosomal enzymes in experimental atherosclerosis, and the effect of cortisone.

Summary Four hydrolytic enzymes present in lysosomes, beta-glucuronidase, acid phosphatase, cathepsin, and aryl sulfatase were studied in the aortas of rabbits with cholesterol-induced atherosclerosis. Each of the enzymes showed a marked increase in activity. It was found in agreement with others that addition of cortisone to the atherogenic diet inhibits the atherosclerosis despite very high levels of serum cholesterol. In the aortas of the cortisone-treated rabbits, the four lysosomal enzymes showed no increases in activity.

Influence of age on aortic cholesterol esterase in rats

Abstract The activity of enzymes which catalyze the synthesis and hydrolysis of cholesteryl esters was studied using acetone dry powder prepared from aortas of normal male rats aged 2, 12 and 24 months. The rates of both synthesis and hydrolysis rose with age but whereas synthetic activity rose (from the 2 month level) by 41% at 12 months and 79% at 24 months, hydrolysis was increased by 345% and 1160% at 12 and 24 months, respectively. These findings augment studies by others who reported that lipolytic activity of rat aorta increases with age.

Metabolism of the Arterial Wall

The enzymes of lipid, carbohydrate, protein and nucleic acid metabolism in normal and arteriosclerotic aorta have been studied (Kirk, 1963). In a survey of enzyme activities, Kirk (1969) pointed out that aortic enzymic activity could not be directly correlated with the severity or type of arteriosclerotic involvement, some activities rising and some falling (Table 1). Since it is not possible to deal with all enzymic activities in the aorta, we shall deal with those relating to that particular component which showed the greatest degree of change, namely, lipid. Smith (1965) has shown that the mucopolysaccharide content of the fatty streak was 115% of that in the normal aorta but in the fibrous and calcified plaque, the mucopolysaccharide content was 96% and 46% of normal. Collagen content was 106%, 166% and 285% that of normal in the fatty streak, fibrous plaque and calcified plaque, respectively. Lipid content rose most strikingly with progression of the lesion in the fatty streak being 312% or normal, 483% in the fibrous plaque and 1112% in the calcified plaque. Among the lipid classes, the most marked increase was that observed in the aortic cholesterol ester. This increase was observed by Windaus (1910) and was further quantitated by Bottcher (1961) and Smith (1965). The phospholipid content of the artery did not vary sharply with age and atherosclerosis, but the sphingomyelin content of human arteries showed a sharp rise in atherosclerotic lesions (Bottcher, 1961).