O-(4-Diazo-3,5-di[125I]iodobenzoyl)sucrose, a novel radioactive label for determining organ sites of catabolism of plasma proteins

Infection of mice with lactate dehydrogenase-elevating virus destroys the subpopulation of Kupffer cells involved in receptor-mediated endocytosis of lactate dehydrogenase and other enzymes

In previous experiments in rats, we have shown that the rapid plasma clearance of a number of clinically important enzymes is due to receptor-mediated endocytosis by Kupffer cells and other resident macrophages. Others have shown that infection of mice with lactate dehydrogenase-elevating virus, a virus that proliferates in macrophages, leads to reduced plasma elimination of these enzymes. This paper integrates these two sets of experiments. Plasma elimination of intravenously injected, radioactively labeled lactate dehydrogenase M4 and mitochondrial malate dehydrogenase in mice was shown to be caused in part by uptake in liver, spleen and bone. Uptake of lactate dehydrogenase M4 by these tissues was, to a large extent, saturable and the two dehydrogenases competitively inhibited each other's clearance. These results suggest that, also in mice, these enzymes are partly cleared from plasma by endocytosis by way of a common receptor on cells (probably macrophages) from liver, spleen and bone marrow. Morphometrical data showed that normal mouse liver contains 23 x 10(6) Kupffer cells/cm3. This number was reduced to about 30% of that of controls 24 hr after infection of mice with lactate dehydrogenase-elevating virus but returned to normal within the next 9 days. The saturable component of uptake of lactate dehydrogenase M4 by liver, spleen and bone had disappeared 24 hr after infection with the virus, and did not return after the Kupffer cell population had recovered. Our findings suggest that lactate dehydrogenase M4 is, to a large extent, removed from the circulation by way of a receptor on a subpopulation of macrophages that is permissive for replication of lactate dehydrogenase-elevating virus.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the inhibitor of the plasminogen activator as the major protein secreted by endothelial rat liver cells

Freshly isolated Kupffer and endothelial liver cells exhibit a rate of 'de novo' protein synthesis which is twice as high per mg cell protein as that of parenchymal liver cells and contribute significantly (7.5% and 5.9%, respectively) to total liver protein secretion. In parenchymal cells the main secretory protein is a 68 kDa protein (containing 19% fo the secreted radioactivity, presumably albumin). In Kupffer cells a 49 kDa protein contains 8% of the secreted radioactivity, while in endothelial liver cells a 55 kDa protein is the most prominent secretory protein (containing 11% of the secreted radioactivity). By aid of a specific antibody the 55 kDa protein was identified as the inhibitor of the plasminogen activator and in the liver this protein was only secreted by the endothelial cells.

Regulation of enzyme levels in the blood. Influence of environmental and genetic factors on enzyme clearance

Since its discovery, lactic dehydrogenase virus (LDV) has remained unique as a model of long-term enzyme elevation due to impairment of enzyme clearance. The present study shows that mice inoculated with silica develop an increase in plasma lactate dehydrogenase (LDH) lasting for at least 6 months and that the enzyme elevation is due, at least in part, to impairment of clearance. The extent of the enzyme elevation is dependent on both the dose and route of silica administration and mice that had received both silica and LDV showed a more profound impairment of LDH clearance than mice that had received silica or LDV alone. Examination of the factors that regulate circulating enzyme levels in normal mice revealed that whereas there was no difference in resting enzyme levels among several inbred strains of mice (BALB/cAnN, NZBWF1/J,B10.D2/nSnN, and A/J mice), when mice were stressed by the administration of an enzyme load, certain inbred strains (BALB/cAnN) cleared the enzyme rapidly and others (B10.D2/nSnN) cleared the enzyme slowly. Moreover, in B10.D2/nSnN mice, enzyme clearance was age-related. When different strains of mice were infected with LDV, LDH levels were substantially higher in the circulation of slow enzyme clearers as compared to rapid enzyme clearers. It is concluded that both environmental and genetic factors influence the clearance of LDH and that impairment of enzyme clearance may be a more important factor than previously suspected in regulating enzyme levels in disease states.

Regulation of liver metabolism by intercellular communication

The regulation of liver metabolism by intercellular communication was assessed by studying the effect of conditioned media of Kupffer and liver endothelial cells on protein synthesis, protein phosphorylation and glycogenolysis in parenchymal cells. Kupffer and endothelial cell-conditioned media enhanced the rate of protein synthesis of parenchymal cells by a factor of 1.7-1.9. The phosphorylation state of only three specific parenchymal cell proteins was influenced by the conditioned media. One, the MW 97,000 band appeared to be phosphorylase and it was found that in parallel with an enhancement of the activity of phosphorylase the glucose output by parenchymal cells could be stimulated. The effects of the conditioned media could be mimicked by prostaglandin E1, E2 and D2, whereas the pretreatment of non-parenchymal cells with aspirin abolished the stimulatory effect of these cells on the glucose output by parenchymal cells. The data indicate that prostaglandins from Kupffer and endothelial cells, mainly PGD2, can influence glucose release from parenchymal cells. The physiological importance of cellular communication was further assessed in a liver perfusion system. The tumor promoting phorbol ester PMA stimulated glycogenolysis in the perfused liver two-fold. This stimulation was blocked by the presence of aspirin. PMA is inactive on isolated parenchymal cells. Addition of PMA to the perfused liver appears to enhance the output of PGD2 in parallel with the stimulation of the glucose output. Addition of prostaglandin D2 itself could also stimulate the glucose output in the perfused liver. Our data indicate that the stimulation of glycogenolysis in the liver by PMA is mediated by non-parenchymal cells which produce PGD2 in response to PMA, leading subsequently to activation of the phosphorylase system in the parenchymal cells. It seems possible also that the tumor-promoting activity of PMA on liver will be mediated by a primary interaction with non-parenchymal cells. It is concluded that the occurrence of intercellular communication inside the liver in response to activation of non-parenchymal cells adds a new mechanism to the complex regulation of liver metabolism which may be relevant under normal and pathological conditions.

Effect of 17 alpha-ethinylestradiol on the catabolism of high-density lipoprotein apolipoprotein A-I in the rat

The in vivo metabolism and tissue sites of catabolism of high-density lipoproteins (HDL), labelled specifically in the apolipoprotein (apo) A-I moiety, were studied in rats treated with 17 alpha-ethinylestradiol (EE) for 5 days. Apo A-I was labelled either with O-(4-diazo-3-[125I]iodobenzoyl)sucrose, a non-degradable labelling compound, or with 131ICl. It was found that EE treatment decreases the serum cholesterol concentration to 10 mg/dl and stimulates the serum decay of apo A-I labelled HDL. The latter effect could be attributed to an increased catabolism of apo A-I labelled HDL in the liver. The increased rates of the serum decay and tissue uptake of apo A-I labelled HDL in EE-treated rats were not affected by a bolus injection of unlabelled human low-density lipoprotein (LDL), administered at the time of the injection of the labelled HDL. When the serum cholesterol concentration was raised to physiological levels by a bolus injection of unlabelled rat HDL, both the serum decay and the tissue uptake of apo A-I labelled HDL were almost completely restored to conditions encountered in control animals. In vitro binding experiments showed that liver membranes obtained from EE-treated rats demonstrated a 6-fold increased specific binding of human 125I-LDL, but virtually unchanged specific binding of rat 125I-HDL, as compared with liver membranes obtained from control rats. It is concluded that rat HDL apo A-I catabolism is hardly mediated by the apo B/E receptor induced by EE treatment.

Effect of 4-aminopyrazolo[3,4-d]pyrimidine on the catabolism of high-density lipoprotein apolipoprotein A-I in the rat

The in vivo turnover and sites of catabolism of O-(4-diazo-3-[125I]iodobenzoyl)sucrose-labelled rat high-density lipoprotein (HDL) apolipoprotein A-I were studied in rats treated for 3 days with 4-aminopyrazolo-[3,4-d]pyrimidine (4APP). It was found that 4APP treatment decreases the serum cholesterol concentration to 6 mg/dl and stimulates the serum decay of labelled HDL. The latter effect could be attributed to an increased catabolism of radioactive HDL apolipoprotein A-I by the liver. When the serum cholesterol concentration was raised to physiological levels by a bolus injection of unlabelled rat HDL, at the time of administration of the labelled HDL, the serum decays and the tissue uptakes of apolipoprotein A-I labelled HDL were identical in 4APP-treated rats and control animals. When a bolus injection of unlabelled human low-density lipoprotein (LDL) was administered to 4APP-treated rats, the serum decay and tissue uptake of apolipoprotein A-I labelled HDL remained rapid. The recovery of radioactivity in the adrenal glands was increased almost 10 fold by 4APP treatment, a phenomenon which was reversed by a bolus injection of unlabelled HDL, but not by injection of unlabelled LDL. It is concluded that treatment of rats with 4APP does not affect the rate of catabolism of rat HDL apolipoprotein A-I, when the serum HDL concentration in the treated animals is restored to physiological levels.

Discrepancies in the catabolic pathways of human and rat high-density lipoprotein apolipoprotein A-I in the rat

The in vivo metabolism in the rat of radioiodinated human and rat high-density lipoprotein was compared with a double-label procedure using 125I and 131I. While rat high-density lipoprotein showed a biphasic serum decay, human high-density lipoprotein was characterized by a monoexponential serum decay. No differences were observed between the serum decay of human high-density lipoprotein-2 and -3 subfractions, isolated by rate zonal ultracentrifugation. The catabolic sites of human and rat high-density lipoprotein were analysed using the lysosomal cathepsin inhibitor leupeptin. Radioiodinated rat high-density lipoprotein was catabolized by the kidneys and by the liver. In contrast, radioiodinated human high-density lipoprotein was catabolized almost exclusively in the liver. No difference in the catabolic sites of human high-density lipoprotein-2 and -3 subfractions was observed. The catabolic sites of human high-density lipoprotein apolipoprotein A-I in the rat were further analysed using the O-(4-diazo-3-[125I]iodobenzoyl) sucrose label. Compared with rat high-density lipoprotein apolipoprotein A-I, the kidneys played a minor role in the catabolism of human high-density lipoprotein apolipoprotein A-I. It is concluded that in the rat the catabolic pathways of the apolipoprotein A-I moieties of rat and human high-density lipoproteins are different, indicating that homologous high-density lipoproteins should be used for the investigation of in vivo metabolism.

125I-glycoconjugate labels for identifying sites of protein catabolism in vivo: effect of structure and chemistry of coupling to protein on label entrapment in cells after protein degradation

Residualizing radioactive labels are designed to remain entrapped within cells following degradation of a carrier protein, and have been used for identification of the tissue and cellular sites of plasma protein catabolism. In this study we describe a convenient synthesis and purification of a series of 125I-labeled glycoconjugates, and an evaluation of their efficiency of retention in liver following degradation of a model carrier protein, asialofetuin. Glycoconjugates were prepared in 65-90% yield by reductive amination of reducing sugars with aromatic amines using NaBH3CN. The products were purified in a single ion-exchange chromatographic step, and then labeled with 125I. The derivatives prepared were mono-and disubstituted lactitol-,cellobiitol-and glucitol-[125I]tyramine and lactitol-[125I]tyrosine. 125I-Glycoconjugates were coupled to asialofetuin using either cyanuric chloride or, for lactose-containing labels, by treatment with galactose oxidase followed by reductive amination with NaBH3CN. Attachment of labels by either procedure did not affect the normal rapid clearance of asialofetuin from the rat circulation nor its uptake and degradation in liver lysosomes. Leakage of 125I-labeled degradation products from cells was measured by following the kinetics of loss of whole-body radioactivity. We observed that degradation products from larger, disubstituted glycoconjugates were retained more efficiently than those from smaller and monosubstituted derivatives, and that glycoconjugates coupled to protein via reductive amination were retained in the body more efficiently than those coupled by cyanuric chloride. Overall, dilactitol-[125I]tyramine coupled to protein by reductive amination was entrapped most efficiently in liver.

Several dehydrogenases and kinases compete for endocytosis from plasma by rat tissues

Plasma contains many enzymes that are probably derived from damaged cells. These enzymes are cleared at characteristic rates. We showed previously that in rats the rapid clearance of alcohol dehydrogenase, lactate dehydrogenase M4 and the mitochondrial and cytosolic isoenzymes of malate dehydrogenase is largely due to endocytosis by macrophages in liver, spleen and bone marrow. We now demonstrate that uptake of each of the enzymes by these tissues is in general decreased by simultaneous injection of a high dose of one of the other dehydrogenases or a high dose of adenylate kinase or creatine kinase. A similar dose of colloidal albumin did not significantly decrease uptake of the four dehydrogenases. Nor was uptake of colloidal albumin, apo-peroxidase from horseradish or multilamellar liposomes influenced by a high dose of mitochondrial malate dehydrogenase. These results indicate that the four dehydrogenases and the two kinases are specifically endocytosed via the same receptor. We suggest that this receptor contains a group, possibly a nucleotide, with affinity for the nucleotide-binding sites of the enzymes.

The sites of degradation of rat high-density-lipoprotein apolipoprotein E specifically labelled with O-(4-diazo-3-[125I]iodobenzoyl)sucrose

O-(4-Diazo-3-[125I]iodobenzoyl)sucrose ([125I]DIBS), a novel labelling compound specifically designed to study the catabolic sites of serum proteins [De Jong, Bouma, & Gruber (1981) Biochem. J. 198, 45-51], was applied to study the tissue sites of degradation of serum lipoproteins. [125I]DIBS-labelled apolipoproteins (apo) E and A-I, added in tracer amounts to rat serum, associate with high-density lipoproteins (HDL) just like conventionally iodinated apo E and A-I. No difference is observed between the serum decays of chromatographically isolated [125I]DIBS-labelled and conventionally iodinated HDL labelled specifically in either apo E or apo A-I. When these specifically labelled HDLs are injected into fasted rats, a substantial [125I]DIBS-dependent 125I accumulation occurs in the kidneys and in the liver. No [125I]DIBS-dependent accumulation is observed in the kidneys after injection of labelled asialofetuin or human low-density lipoprotein. It is concluded that the kidneys and the liver are important sites of catabolism of rat HDL apo E and A-I.

Complexes of rat alpha 1-macroglobulin and subtilisin are endocytosed by parenchymal liver cells

Rat alpha 1-macroglobulin was isolated from plasma. Gel electrophoresis of the denatured and reduced protein showed two bands, with Mr values of 163 000 and 37 000. The large subunit contained an autolytic site. This subunit was also split after reaction of the macroglobulin with trypsin. Electron microscopy showed that the macroglobulin changed towards a more compact conformation after reaction with this proteinase. Subtilisin, or alpha 1-macroglobulin, was labelled with a sucrose-containing radio-iodinated group that stays in lysosomes after endocytosis and breakdown of the tagged protein. After intravenous injection into rats, alpha 1-macroglobulin was cleared from plasma with first-order kinetics, showing a half-life of about 9 h, whereas complexes of alpha 1-macroglobulin and subtilisin were cleared with half-lives of only 3 min. Liver contained about 60% of the label at 30 min after injection of complexes. About 90% of the liver radioactivity was found in parenchymal cells isolated after perfusion of the liver with a collagenase solution. Subcellular fractionation indicated a lysosomal localization of the complexes. We conclude that endocytosis by parenchymal liver cells is the major cause of the rapid clearance of alpha 1-macroglobulin-proteinase complexes from plasma.

Application of O-(4-diazo-3-[125I]iodobenzoyl)sucrose for the detection of the catabolic sites of low density lipoprotein

The sites of degradation of human low density lipoprotein (LDL), are analyzed using the novel labelling compound O-(4-diazo-3-[125I]iodobenzoyl)sucrose (D125IBS). The decay from rat serum of D125IBS-labelled LDL is identical to the serum decay of conventionally iodinated (ICI method) LDL. The radioactivity derived from D125IBS-labelled LDL accumulates predominantly in the liver after intravenous injection and the hepatic radioactivity remains associated with the lysosomal compartment for an extended period of time, when compared to the radioactivity derived from conventionally iodinated LDL. It is concluded that the D125IBS labelling procedure is an interesting new tool to study the sites of catabolism of serum lipoproteins.

Organ specific metabolism of low density lipoprotein and high density lipoprotein

Recent data indicate that the kidneys are the most important organ for the catabolism of HDL-apolipoproteins in the rat. This catabolic process is probably initiated by a specific HDL receptor in kidney membranes. However, it is unlikely that receptor-mediated endocytosis of intact HDL is involved in HDL-apolipoprotein degradation by kidneys, since HDL-cholesteryl esters are not taken up by the kidneys in vivo. It is proposed that the kidneys catalyze the first rate-limiting step in the degradation of HDL by "stripping" and degradation of a part of the HDL-apolipoproteins. In vivo metabolism of HDL-cholesteryl esters and LDL occurs predominantly in the liver and not to any significant extent in the kidneys.

Tissue locations for the turnover of radioactively labeled rat orosomucoid in vivo

Tissues involved in the turnover of rat serum orosomucoid were identified by methods designed to cause lysosomal trapping of radiolabel at the sites of glycoprotein degradation. 125I-, [3H]Raffinose-, and [1-14C]glucosamine-labeled orosomucoid exhibited serum half-lives of 20, 20, and 27 h when injected intravenously into rats. As expected, the asialo derivative of [3H]raffinose-labeled rat orosomucoid was lost very rapidly from the circulation and recovered quantitatively in the liver within 30 min. At 50 h after injection of [3H]raffinose-asialo-orosomucoid the liver retained 38% of the radioactivity while the remainder was found in the gastrointestinal tract and urine. Chromatography of the urine on Bio-Gel P-4 revealed a single radioactive product that eluted similar to raffinose-lysine. The same material was found in the liver. This ability of the [3H]raffinose label to resist metabolic disposal was used to evaluate tissue catabolism of native rat orosomucoid. Comparison of the tissue radioactivity in experiments using 125I- and [3H]raffinose-labeled derivatives of the nondesialylated glycoprotein showed kidney, liver, and muscle to be most active in 3H accumulation. However, the [3H]raffinose metabolites excreted in the urine was markedly different from those produced from asialo-orosomucoid and in contrast there was minimal loss of label to the gastrointestinal tract from the native substrate. Leupeptin, an inhibitor of lysosomal thiol cathespins, was administered continuously to rats by a subcutaneous osmotic pump. At 24 h after injection of 125I-orosomucoid, leupeptin-treated rats showed a net 16% increase in tissue radioactivity above sham-operated animals and a corresponding decrease occurred in the radioactivity associated with the gastrointestinal tract and urine. Tissues that exhibited increases in radioactivity were kidney, muscle, liver, and hide. The different behavior of labeled native and asialo-orosomucoids suggests that the hepatic galactose receptor system plays, at most, a limited role in maintaining homeostasis of the native glycoprotein.

A radioiodinated, intracellularly trapped ligand for determining the sites of plasma protein degradation in vivo

We recently developed a general method for determining tissue sites of degradation of plasma proteins in vivo that made use of covalently attached radioactive sucrose. On degradation of the protein, the sucrose remained trapped in the cells as a cumulative marker of protein degradation. The method described here depends on the same principles, but uses an adduct of cellobiose and tyramine that is radioiodinated to high specific radioactivity and then covalently attached to protein. Use of the radioiodinated ligand increases the sensitivity of the method at least 100-fold and allows simplified tissue analysis. Proteins derivatized with the radioiodinated ligand were recognized as underivatized proteins both in vitro and in vivo. On degradation of derivatized low-density lipoprotein, the rate of leakage from cultured fibroblasts was only 5% during 24 h. Similarly, on injection of labelled proteins into rats and rabbits, urinary excretion of the label was in all cases less than 10% of total labelled catabolic products recovered 24 h after injection. Examination of the tissue contents of label at two times after injection of labelled asialofetuin or apolipoprotein A1 in rats, and asialotransferrin in rabbits showed that the label did not detectably redistribute between tissues after initial uptake and catabolism; a significant leakage from liver was quantitatively accounted for by label appearing in gut contents and faeces. A simple double-label method was devised to provide a correction for intact protein in trapped plasma, the extravascular spaces, and within cells. By using this method it becomes unnecessary to fractionate tissue samples.

Plasma clearance and endocytosis of cytosolic malate dehydrogenase in the rat

1. Pig heart cytosolic malate dehydrogenase was radiolabelled with O-(4-diazo-3,5-di-[125I]iodobenzoyl)sucrose and intravenously injected into rats. Enzyme activity and radioactivity were cleared from plasma identically, with first-order kinetics, with a half-life of about 30 min. 2. The tissue distribution of radioactivity was determined at 2 h after injection. All injected radioactivity was recovered from the tissues. A high percentage of the injected dose was found in liver (37%), spleen (6%) and bone including marrow (19%). 3. Radioactivity in liver and spleen increased up to 2 h after injection and subsequently declined, with a half-life of about 20 h. 4. After differential fractionation of liver, radioactivity was largely found in the mitochondrial and lysosomal fraction. 5. Liver cells were isolated 1 h after injection of labelled enzyme. We found that Kupffer cells, endothelial cells and parenchymal cells had endocytosed the enzyme at rates corresponding to 2725, 94 and 63 ml of plasma/day per g of cell protein respectively. 6. Radioautography indicated that in spleen and bone marrow the enzyme is mainly taken up by macrophages. 7. Internalization of the enzyme by liver, spleen and bone marrow was saturable. This indicates that the enzyme is taken up in these tissues by adsorptive endocytosis. 8. The present results closely resemble those obtained previously for the mitochondrial isoenzyme of malate dehydrogenase and for lactate dehydrogenase M4. Since those enzymes are positively charged at physiological pH, whereas cytosolic malate dehydrogenase is negative, net charge cannot be the major factor determining the rate of uptake of circulating enzymes by reticuloendothelial macrophages, as has been suggested in the literature [Wachsmuth & Klingmüller (1978) J. Reticuloendothel. Soc. 24, 227-241].

Endocytosis and breakdown of mitochondrial malate dehydrogenase in the rat in vivo. Effects of suramin and leupeptin

1. The plasma clearance of intravenously injected 125I-labelled mitochondrial malate dehydrogenase (half-life 7 min) was not influenced by previous injection of suramin and/or leupeptin (inhibitors of intralysosomal proteolysis). 2. Pretreatment with both inhibitors considerably delayed degradation of endocytosed enzyme in liver, spleen, bone marrow and kidneys. 3. The tissue distribution of radioactivity was determined at 30 min after injection, when only 3% of the dose was left in plasma. All injected radioactivity was still present in the carcass. The major part of the injected dose was found in liver (49%), spleen (5%), kidneys (13%) and bone, including marrow (11%). 4. Liver cells were isolated 15 min after injection of labelled enzyme. We found that Kupffer cells and parenchymal cells had endocytosed the enzyme at rates corresponding to 9530 and 156 ml of plasma/day per g of cell protein respectively. Endothelial cells do not significantly contribute to uptake of the enzyme. 5. Uptake by Kupffer cells was saturable, whereas uptake by parenchymal cells was not. This suggests that these cell types endocytose the enzyme via different receptors. 6. Previous injection of carbon particles greatly decreased uptake of the enzyme by liver, spleen and bone marrow.

Use of radioactive glucosamine in the perfused rat liver to prepare alpha 1-acid glycoprotein (orosomucoid) with 3H- or 14C-labelled sialic acid and N-acetylglucosamine residues

1. A method was developed whereby [1-14C]glucosamine was used in a perfused rat liver system to prepare over 2 mg of alpha 1-acid glycoprotein with highly radioactive sialic acid and glucosamine residues. 2. The liver secreted radioactive alpha 1-acid glycoprotein over a 4-6 h period, and this glycoprotein was purified from the perfusate by chromatography on DEAE-cellulose at pH 3.6. 3. The sialic acid on the isolated glycoprotein had a specific radioactivity of 3.1 Ci/mol, whereas the glucosamine-specific radioactivity was 4.3 Ci/mole. The latter amino-sugar residues on the isolated protein were only 13-fold less radioactive than the initially added [1-14C]glucosamine. Orosomucoid with a specific radioactivity of 31.3 microCi/mg of protein was obtainable by using [6-3H]glucosamine. 4. The amino acid composition of the purified orosomucoid was comparable with that found by others for the same glycoprotein isolated from rat serum. A partial characterization of the carbohydrate structure was done by sequential digestion with neuraminidase, beta-D-galactosidase and beta-D-hexosaminidase. 5. Many other radioactive glycoproteins were found to be secreted into the perfusate by the liver. Thus this experimental system should prove useful for obtaining other serum glycoprotein with highly radioactive sugar moieties.

Endocytosis of lactate dehydrogenase isoenzyme M4 in rats in vivo. Experiments with enzyme labelled with O-(4-diazo-3,5-di[125I]iodobenzoyl)sucrose

1. Pig lactate dehydrogenase isoenzyme M4 was labelled with O-(4-diazo-3,5-di[125I]iodobenzoyl)sucrose and injected intravenously into rats. Previous work has shown that this label does not influence the clearance of the enzyme (half-life about 26 min) and that it is retained within the lysosomes for several hours after endocytosis and breakdown of the protein [De Jong, Bouma & Gruber (1981) Biochem. J. 198, 45--51]. 2. The distribution of the radioactivity over a large number of tissues was determined 2 h after injection. A high percentage of the injected dose was found in liver (41%), spleen (10%) and bone including marrow (21%). 3. Autoradiography indicated uptake of the enzyme mainly by Kupffer cells of the liver, by spleen macrophages and by bone marrow macrophages. 4. Liver cells were isolated 1 h after injection of the enzyme. Kupffer cells, endothelial cells and parenchymal cells were found to endocytose the enzyme at rates corresponding to 4230, 35 and 25 ml of plasma/day per g of cell protein, respectively. 5. Previous injection of carbon particles greatly reduced the uptake of the enzyme by liver and spleen, but the uptake by bone marrow was not significantly changed.

Plasma clearance and endocytosis of mitochondrial malate dehydrogenase in the rat

1. Pig mitochondrial malate dehydrogenase was labelled with 125I and intravenously injected into rats. Enzyme activity and radioactivity were cleared from plasma identically, with first-order kinetics, with a half-life of only 7 min. 2. Radioactivity accumulated in liver, spleen, bone (marrow) and kidneys, reaching maxima of 3 1, 4, 6 and 9% of the injected dose respectively, at 10 min after injection. 3. Our data allow us to calculate that in the long run 59, 5, 11 and 13% of the injected dose is taken up and subsequently broken down by liver, spleen, bone and kidneys respectively. 4. Differential fractionation of liver showed that the acid-precipitable radioactivity was mainly present in the lysosomal and microsomal fractions, suggesting that the endocytosed protein is transported via endosomes to lysosomes, where it is degraded. 5. Radioautography of liver and spleen suggested that the labelled protein was taken up by macrophages of the reticuloendothelial system. 6. Mitochondrial malate dehydrogenase is probably internalized in liver, spleen and bone marrow by adsorptive endocytosis, since uptake of the enzyme of these tissues is saturable.