Chylomicron injection

While chylomicrons in lymph contain apoA-I as a major apolipoprotein, plasma chylomicrons do not normally contain apoA-I, and apoA-I appears to transfer from chylomicrons to HDL in plasma (A23, P4, S8, T5). Parks and Rudel compared the kinetic fates of labeled apoA-I and apoA-II from lymph chylomicrons in monkeys (P4). The two apolipoproteins behaved differently when injected into plasma. ApoA-II appeared to be almost instantaneously transferred from injected chylomicrons into HDL, while the tracer apoA-I specific activity rose in HDL for 1-3 hours after chylomicron injection, before falling at a rate identical to that of autologous HDL apoA-I tracer. These and other findings suggest that some chylomicron apoA-I transferred to chylomicron remnants or to disks or vesicles of redundant surface material released from chylomicrons, or remained free in solution, rather than immediately transferring to HDL. 

Chylomicron metabolism. Smoke, administered to rats injected intravenously with C- and H-labeled chylomicrons, produced no difference in the initial plasma clearance time of labeled chylomicrons between smoke-treated and control animals. Hepatic uptake of chylomicron cholesterol was slower in smoke-treated animals than in controls. More labeled chylomicrons remained in the heart of smoke-treated rats than controls . 

Whereas LPL predominantly hydrolyzes triglycerides in chylomicrons and VLDL, it has been shown that HL primarily hydrolyzes triglycerides and phospholipids from small VLDL, IDL, and HDL [82]. Like LPL, HL binds to the endothelium through heparan sulfate proteoglycans and is released upon heparin administration because of its higher affinity for heparin than for the endogenous heparan sulfate proteoglycans. Intravenous injection of a heparin bolus displaces the HL enzyme into postheparin plasma, where its activity can be quantified. 

The chylomicron remnant particles themselves, derived from lipolysis of the larger chylomicrons (cf. above), contain the residual triglyceride and all of the cholesterol and cholesterol ester from the original chylomicrons. This lipid composition of the chylomicron remnant particles is similar to the above-described lipid composition of both LDL particles (cf. Section 14.1) and LCM (cf. Section 12.1). Based upon this molecular similarity, it appears reasonable to expect that injected LCM could also readily bind apo E (i.e., as an alternative to apo B) in the bloodstream. In this case, the proposed LCM binding of apo E should influence the subsequent biodistribution of those LCM via two endocytic pathways specifically, one pathway mediated by the LDL receptor (a.k.a. apo B,E receptor ) (cf. Section 14.1) and the other pathway mediated by the LRP, since both receptor types have a high affinity for apo E (cf. above). 

ApoE-containing HDL, obtained by heparin-Sepharose affinity chromatography, contains apoA-I, apoA-II, and apoC, as well as apoE (W12, W18). Clearance frou. the plasm, appears to be dependent on a specific hepatic receptor for apoE, which binds apoE-containing HDL and chylomicron remnants, but not other lipoproteins (H35, Mil, S28). Canine apoE-HDLC, with apoE as the only detectable apoprotein, is cleared from the plasma very rapidly by the liver (more than 90% in the first 20 minutes after intravenous injection) (M15). In adult man, dogs, and swine the apoE receptor numbers do not seem to be significantly reduced by cholesterol feeding (H35, Mil). 

Type I lipoproteinemia is generally caused by the inability of the organism to clear chylomicrons. The problem may be defective ApoC-II or a defective lipoprotein lipase. Very often, chylomicron clearance may be affected by injection of heparin, which apparently releases hepatic lipase from the liver into the circulation. ApoE disorders may be associated with type III lipoproteinemia, in which clearance of IDL is impeded. Increases in circulatory LDL are usually caused by a decrease in tissue receptors specific for ApoB-100. An extreme case of type Ha hyperlipoproteinemia is familial hypercholesterolemia, in which serum cholesterol levels may be as high as 1000 mg/dL and the subjects may die in adolescence from cardiovascular disease. There is total absence of ApoB-100 receptors. Mild type Ila and lib lipoproteinemias are the most commonly occurring primary lipoproteinemias in the general population. 

Critical to vitamin D3 action is its further metabolic conversion to more active compounds (Figure 1.3). Via its transport by DBP, vitamin D3 accumulates in the liver [48]. In rats, as much as 60-80% of an injected or oral dose of vitamin D3 locates to the liver [49-51], Intestinal absorption of vitamin D3 is in association with the chylomicron fraction via the lymphatic system. Vitamin D3 is delivered to the liver in blood from the thoracic duct only a few hours post ingestion [44], A specific portion of hepatic vitamin D3 in the rat is converted to 25-OH-D3 by a 25-hydroxylase system in the endoplasmic reticulum of hepatocytes [52, 53]. This enzyme (Km 10"8 M) is regulated to an extent by 25-OH-D3 and its metabolites. Higher concentrations of vitamin D3 are handled by a second 25-hydroxylase located in liver mitochondria [54], This enzyme, also known as CYP27, 27-hydroxylates cholesterol and thus appears less discriminating than the microsomal 25-OHase which does not use cholesterol as substrate [55, 56]. In humans, however,... 

Sterile parenteral oil-in-water emulsions have been used extensively for over 40 years for the intravenous administration of fats, carbohydrates, and vitamins to debilitated patients. Several vegetable oil-in-water emulsions are now available commercially with droplet sizes similar to that of chylomicrons (approximately 0.5-2 pm), the natural fat droplets in the blood that transport ingested fats to the lymphatic and circulatory systems (Table 1). More recently, such emulsions have been employed as intravenous carriers for poorly water-soluble lipophilic drugs such as vitamin K (e.g.. Sterile Phytonadione Injection U.S.P.) diazepam... 

To prepare pHJvitamin A-labeled chylomicrons (Green et ai, 1993), [ Hjretinol or retinyl acetate is administered intraduodenally to thoracic lymph duct-cannulated donor rats and lymph is collected at 4°C. Chylomicrons containing mainly pHJretinyl esters can be isolated from lymph by preparative ultracentrifugation for administration to recipient rats. Alternatively, aliquots of whole lymph can be injected to minimize handling of the dose (see below). Then the proportions of total lymph radioactivity and vitamin A mass in chylomicrons (typically >8590%) can be determined analytically. In either case, lymph preparations should be used for in vivo studies within 12 days of collection. 

Even when care is taken to handle the doses carefully, we have found that 215% of the tracer in the case of pHjretinol-labeled plasma, and up to 40% in the case of isolated pHJretinyl ester-labeled chylomicrons, acts nonphysiologically when preparations are injected into recipient rats. That is, a variable fraction of the dose (the nonphysiological component) is cleared from plasma within a few minutes. Presumably nonphysiological... 

Redgrave, T. G., Ly, H. L., Quintao, E. C. R., Ramberg, C. F., and Boston, R. C. (1993). Clearance from plasma of triacylglycerol and cholesteryl ester after intravenous injection of chylomicron-like emulsions in rats and man. Biochem. J. 290, 843-847. 

Attention was first drawn to this type of enzyme by Hahn in 1943 (367), who showed that injections of heparin into the blood of an animal with a high level of chylomicrons led to rapid clearing of the plasma, and that this was a result of a lipase-type action. Adition of... 

After injection of a fat emulsion an associate formatiw with apolipoproteins (18) could be delected, which makes the analogy to chylomicrons (%. 110,128,129) seem logical. On the other hand, the foreign body character of the fat emulsions must be taken into consideration. 

Much information is available about the metabolism of chylomicron cholesteryl esters taken up by the liver in association with the chylomicron remnant. This information may be relevant to the issue of chylomicron retinyl ester metabolism in the liver, about which much less direct information is on hand. Hepatic uptake of chylomicron cholesteryl esters occurs without hydrolysis of the cholesteryl esters (Goodman, 1965 (Juarfordt and Goodman, 1967 Stein et al., 1969). In studies with chylomicrons containing doubly labeled cholesteryl esters injected intravenously into rats, Quarfordt and Goodman (1967) observed that 80-90% of the chylomicron cholesteryl esters were removed by the liver without hydrolysis. In the liver, the newly absorbed cholesteryl esters underwent slow but extensive hydrolysis, to the extent of about 60% after 1 h and about 85-90% after 3.5 h. Subsequent to hydrolysis, most of the labeled free cholesterol slowly left the liver and was extensively redistributed in die body. Thus, 24 h later, only 20-28% of the labeled cholesterol found in the entire animal body was present in the liver. Since newly absorbed retinol, which is retained in the liver, is only mobilized slowly (see below), it is clear that following ester hydrolysis the hepatic metabolism of chylomicron cholesterol and retinol diverge in a major way. 

The liver plays the major role in the body in the uptake and metabolism of newly absorbed retinol (retinyl esters). Thus, retinyl esters appear to remain almost completely with the hydrophobic core of the chylomicron during its extrahepatic conversion to a remnant particle, and then to be taken up by the liver in association with the chylomicron remnant. A quantitative and detailed study of these processes was reported by Goodman et al. (1965). Chylomicrons containing newly absorbed labeled retinol (93% as retinyl esters) were injected intra-... 

Repletion of retinol-deficient rats can also be effectively achieved by the intravenous injection of retinol dispersed in a 20% Tween 40 solution (Smith et al., 1980 Fig. 4). Such an injection produces a rapid, dose-related increase in the serum concentration of RBP. The changes in serum RBP levels seen after the injection of retinol in a 20% Tween 40 solution closely resembled those previously seen after the injection of vitamin A (retinyl esters) in association with lymph chylomicrons. However, the amount of retinol required to stimulate the secretion of a given amount of RBP from the liver was about two to three times that required when retinol (retinyl esters) was injected in chylomicrons. As discussed by Smith et al. (1980), this quantitative difference is probably due to differences in the tissue distribution pattern of retinol when injected in the Tween 40 solution, compared to its administration in the form of chylomicrons. 

The uptake of particles has previously been thought to be the task of the reticuloendothelial cells. It could actually be shown that exogenous cholesterol enters Kupffer cells (Friedman et al. 1954) but this process does not seem to be essential for the assimilation of triglycerides nor is it quantitatively important (van den Bosch et al. 1961). Blocking the Kupffer cells by injections of colloidal carbon did not interfere with the removal of fat and its deposition in the parenchymal cells of the liver (Waddell et al. 1954). It seems to be mainly the function of the parenchymal cells to assimilate and metabolize chylomicron triglycerides (DiLxjzio 1960) whereas artifical fat emulsions are also taken up by the reticuloendothelial cells of the liver. 

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