Peroxisomes and peroxisomal functions in muscle. Studies with muscle cells from controls and a patient with the cerebro-hepato-renal (Zellweger) syndrome

In the present study we investigated peroxisomal functions in cultured human muscle cells from control subjects and from a patient with the Zellweger syndrome, a genetic disease characterized by the absence of morphologically distinguishable peroxisomes in liver and kidney. In homogenates of cultured muscle cells from control subjects, catalase is contained within subcellular particles, acyl-CoA:dihydroxyacetonephosphate acyltransferase activity is present and palmitoyl-CoA can be oxidized by a peroxisomal beta-oxidative pathway; these findings are indicative of the presence of peroxisomes in the cells. In homogenates of cultured muscle cells from the patient with the Zellweger syndrome, acyl-CoA:dihydroxyacetonephosphate acyltransferase activity was deficient, peroxisomal beta-oxidation of palmitoyl-CoA was impaired and catalase was not particle-bound. These findings indicate that functional peroxisomes are absent in muscle from patients with the Zellweger syndrome. We conclude that cultured human muscle cells can be used as a model system to study peroxisomal functions in muscle and the consequences for this tissue of a generalized dysfunction of peroxisomes.     

Characterization of glucocerebrosidase in peripheral blood cells and cultured blastoid cells

We have characterized glucocerebrosidase in various cell types of peripheral blood of control subjects and in cultured human blastoid cells. The intracellular level of glucocerebrosidase in cultured blastoid cells (10-30 nmol substrate hydrolyzed/h.mg protein) resembles closely values observed for leukocyte cell types and various tissues and is significantly lower than that observed in cultured fibroblasts (150-500 nmol substrate hydrolyzed/h.mg protein). Glucocerebrosidase is extracted from leukocyte cell types and cultured blastoid cells almost exclusively in a monomeric, nonactivated form with enzymatic properties identical to those of the tissue enzyme. In contrast, extracts of platelets are rich in an aggregated, activated form of the enzyme. Glucocerebrosidase in blood cells and cultured blastoid cells is heterogeneous with respect to Mr and pI due to a heterogeneous oligosaccharide composition of the enzyme. The different forms seen represent intermediates in the biosynthesis and maturation of the enzyme. Blastoid cells should thus be an attractive model system for studying the natural history of glucocerebrosidase in a cell type related to those cells involved in the pathology of Gaucher disease.     

Comparison of the properties of a soluble form of glucocerebrosidase from human urine with those of the membrane-associated tissue enzyme

Human urine contains a soluble form of glucocerebrosidase, an enzyme associated with the lysosomal membrane in cells and tissues. Urinary glucocerebrosidase is identical to the enzyme extracted from tissues with respect to the following parameters: Km for natural and artificial substrates, inhibition by conduritol B-epoxide, and stimulation by taurocholate. The enzyme is greater than 90% precipitable by polyclonal anti-(placental glucocerebrosidase) antiserum. Upon isoelectric focussing of urinary glucocerebrosidase multiple peaks of activity were observed. Partial deglycosylation (removal of sialic acid, N-acetylglucosamine and galactose) of the urinary enzyme increased the isoelectric point to a value identical to that of the main form found after partial deglycosylation of the placental enzyme. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate followed by immunoblotting, the immunopurified urinary enzyme shows the same molecular mass forms as the enzyme immunopurified from brain and kidney. In placenta the apparent molecular mass is somewhat higher but upon removal of sialic acid, N-acetylglucosamine and galactose the urinary and the placental enzyme show identical molecular masses of 57 kDa. We conclude that the enzymes extracted from urine and tissue are identical and that differences in apparent molecular mass and isoelectric point are probably due to heterogeneity in the oligosaccharide moieties of the molecules.     

Acyl-CoA:dihydroxyacetone phosphate acyltransferase in human skin fibroblasts: study of its properties using a new assay method

In relation to the finding that human skin fibroblasts are capable of de novo either phospholipid biosynthesis, we have studied the properties of acyl-CoA:dihydroxyacetone phosphate acyltransferase in fibroblast homogenates using a new assay method. The results indicate that the acylation of dihydroxyacetone phosphate shows an optimum at pH 5.5 with a broad shoulder of activity up to pH 6.4 and a decline in activity up to pH 8.2. At pH 5.5 the acyltransferase accepts dihydroxyacetone phosphate, but not glycerol 3-phosphate as a substrate. Furthermore, the transferase activity was found to be membrane-bound and inactivated by Triton X-100 at concentrations above 0.025% (w/v). Similar properties have been described for the enzyme as present in rat-liver and guinea-pig liver peroxisomes. These data, together with the finding that acyl-CoA:dihydroxyacetone phosphate acyltransferase is deficient in cultured skin fibroblasts from patients without peroxisomes (Zellweger syndrome), suggest that in cultured skin fibroblasts the enzyme is primarily located in peroxisomes.     

Peroxisomal beta-oxidation enzyme proteins in the Zellweger syndrome

The absence of peroxisomes in patients with the cerebro-hepato-renal (Zellweger) syndrome is accompanied by a number of biochemical abnormalities, including an accumulation of very long-chain fatty acids. We show by immunoblotting that there is a marked deficiency in livers from patients with the Zellweger syndrome of the peroxisomal beta-oxidation enzyme proteins acyl-CoA oxidase, the bifunctional protein with enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities and 3-oxoacyl-CoA thiolase. Using anti-(acyl-CoA oxidase), increased amounts of cross-reactive material of low Mr were seen in the patients. With anti-(oxoacyl-CoA thiolase), high Mr cross-reactive material, presumably representing precursor forms of 3-oxoacyl-CoA thiolase, was detected in the patients. Catalase protein was not deficient, in accordance with the finding that catalase activity is not diminished in the patients. Thus in contrast to the situation with catalase functional peroxisomes are required for the stability and normal activity of peroxisomal beta-oxidation enzymes.     

Kinetic analysis of short-term effects of alpha-agonists on gluconeogenesis in isolated rat hepatocytes

Isolated hepatocytes from fasted rats were perifused with glycerol as gluconeogenic substrate. Stimulation of gluconeogenesis with phenylephrine (10(-5) M) as alpha-adrenergic agonist consisted of two distinct phases. The first phase was a transient stimulation of gluconeogenesis and was accompanied by transient changes in cytosolic and mitochondrial redox state; this phase was abolished by the transaminase inhibitor aminooxyacetate. The second phase was a stable stimulation of less magnitude, without change in redox state and insensitive to addition of aminooxyacetate. It is concluded that the first phase is due to a transient enhancement of flux through the malate/aspartate shuttle and that the stable phase is probably due to a stimulation of mitochondrial glycerol-3-phosphate dehydrogenase and glycerol kinase.     

Nutritional influences on the distribution of the urea cycle: intermediates in isolated hepatocytes

After the urea cycle was proposed, considerable efforts were put forth to identify critical intermediates. This was then followed by studies of dietary and nutritional control of urea cycle enzyme activity and allosteric effectors of urea cycle enzymes. Correlation of urea cycle enzyme activity with isolated cell experiments indicated conditions where enzyme activity would be rate limiting. At physiological levels of ammonia the activation of carbamoyl-phosphate synthetase (EC 6.3.4.16) by N-acetylglutamate (NAG) is important. Various levels of NAG corresponded well with changes in the rate of citrulline and urea synthesis. Arginine was found to be an allosteric activator of N-acetylglutamate synthetase (EC 2.3.1.1). Therefore, it was possible that the rate of carbamoyl phosphate synthesis was dependent on the level of urea cycle intermediates, particularly arginine. Evidence for arginine in the regulation of NAG synthesis is not as clear as for NAG on carbamoyl phosphate synthetase I. The concentration of hepatic arginine is not necessarily an indication of the mitochondrial concentration. Only mitochondrial arginine stimulates the N-acetylglutamate synthetase. Recent studies indicate that the mitochondrial concentration of arginine is higher than the cytosolic concentration and is well above the Ka for N-acetylglutamate synthetase. Therefore, it appears that changes in arginine concentration are not physiologically important in regulating levels of NAG. However, it is possible that responses to the effector may vary with time after eating, and it may be this responsiveness that controls the level of NAG and thereby urea synthesis.     

Heterogeneity of glucagon receptors of rat hepatocytes: a synthetic peptide probe for the high affinity site

A glucagon analog with the following sequence has been synthesized: His- Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg -Leu-Gln-Glu-Phe-Leu-Gln-Trp-Ala-Leu-Gln-Thr. When interacting with rat hepatocytes, the analog mimics, in part, the activities of glucagon in receptor binding and inhibition of carbohydrate incorporation into glycogen. Comparison of the binding of the analog with that of glucagon demonstrates the existence of two distinct homogeneous populations of glucagon receptors. The synthetic analog acts as a specific probe for those receptors that have a high affinity for glucagon.     

Receptor binding and cell-mediated metabolism of [125I]monoiodoglucagon by isolated canine hepatocytes

We have developed a reverse-phase HPLC method to purify 125I-labeled products resulting from the chloramine-T-based iodination of glucagon and have used the products [(125I)iodoTyr13]glucagon, [(125I)iodoTyr10,13]glucagon, and [(125I)iodoTyr10]glucagon) to study the receptor binding of glucagon and the cell-mediated metabolism of the hormone by isolated canine hepatocytes. The extent of binding of the three labeled glucagons to cell receptors differed at steady state (8.5, 11.9, and 12.6% of the three peptides, respectively, becoming cell-associated), but each of the labeled glucagons approached steady state binding at the same rate. Further, unlabeled glucagon competed for the binding of each of the labeled peptides in parallel under steady state conditions, and each of the peptides showed potent activity in inhibiting [14C]fructose incorporation into glycogen. Gel filtration of the acetic acid-extracted, cell-associated products of radiolabeled glucagon binding revealed 10-20% of the material as a shoulder on the descending limb of the peak of hormone for each of the three labeled peptides. Trypsin digestion of the lower molecular weight peptide derived from [(125I)iodoTyr13]glucagon resulted in a fragment containing residues 13 to 17 as the only detectable radiolabeled product. On the other hand, trypsin digestion of the analogous peptide derived from [(125I)iodoTyr10]glucagon revealed, in addition to the radiolabeled fragment containing residues 1 to 12, a major fragment identified by radiosequence analysis to contain residues 4 to 12 and a minor fragment identified to contain residues 7 to 12. We conclude that (a) notwithstanding apparent differences in affinities exhibited by [(125I)iodoTyr13]glucagon, [(125I)iodoTyr10,13]glucagon, and [(125I)iodoTyr10]glucagon for binding to canine hepatocytes, the interactions of all three peptides with the glucagon receptor are functionally equivalent, and (b) the cell-mediated metabolism of receptor-bound glucagon involves the formation of hormone-derived peptides in which the biologically important NH2-terminal region of the hormone has been modified by limited proteolytic cleavage.