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Biomedical subjects

A Jochen

Publications and source records attributed to A Jochen.

7 recordsLinked to original sources
PubMed1997

Age-related changes in male gonadal function. Implications for therapy.

In contrast with women, who experience a complete and abrupt cessation of ovarian function during the menopause, aging men largely maintain their testicular androgen production. Nevertheless, most cross-sectional studies indicate that there is a partial decrease in testosterone levels with aging, although this has not been confirmed by other studies. The disparity among studies stems from differences in study design, patient numbers, assay techniques and inclusion criteria. Proposed mechanisms for an age-associated decline in testosterone production include: (i) defects in the hypothalamic-pituitary-testicular axis; (ii) an increase in sex hormone binding globulin levels; (iii) environmental factors; (iv) medication use; and (v) chronic illness. The potential beneficial effects of testosterone replacement therapy in hypogonadal men include increased bone density, increased muscle strength, an improved feeling of well-being and an improved metabolic profile. These benefits need to be weighted against the potential risks of androgen therapy, such as erythrocytosis, sleep apnoea, and the stimulation of benign prostatic hypertrophy or an occult prostate malignancy. Consequently, androgen replacement should be used with caution in elderly men with hypogonadism until the results of well-controlled prospective studies are available.

Aged↗
PubMed1993

Purification of the major substrate for palmitoylation in rat adipocytes: N-terminal homology with CD36 and evidence for cell surface acylation.

We previously reported that incubating rat adipocytes with [3H]palmitate predominantly labeled an 85-kDa protein. The labeling was more intensive in the presence of insulin and had characteristics consistent with covalent fatty acylation (Jochen et al. 1991. Biochem. Biophys. Res. Commun. 177: 797-801). In order to determine the significance of this finding we purified the 85-kDa protein, determined its N-terminal sequence, and further characterized its interactions with long-chain fatty acids. The [3H]palmitate-labeled 85-kDa protein was purified from rat adipocyte membranes using the following sequence of procedures: i) affinity chromatography with wheat germ agglutinin-agarose, ii) ion exchange chromatography with DEAE-Sepharose, and iii) SDS-polyacrylamide gel electrophoresis. The resulting labeled protein was sequenced through 30 amino acid residues. With the exception of one conserved substitution, the sequence was identical to CD36 (platelet membrane glycoprotein IV). Further characterization of the 85-kDa protein revealed it was heavily N-glycosylated and possessed a cell surface domain. Labeling of the 85-kDa protein with palmitate was compared in control cells, insulin-treated cells, and cells whose energy was depleted with 2,4-dinitrophenol. Insulin and energy depletion increased labeling approximately 3-fold and 12-fold, respectively. Labeling performed in the presence of energy depletion possessed all of the characteristics of covalent protein acylation. In addition, there was a close association between the ability of energy depletion to increase labeling of the 85-kDa protein and its ability to inhibit depletion of [3H]palmitate from the extracellular incubation media. These results suggest that the major substrate for fatty acylation in adipocytes is a cell surface membrane protein related to CD36 and that this acylation has the unusual properties of being independent of intracellular metabolic energy and of occurring on an exofacial epitope of the protein.

2,4-Dinitrophenol↗
PubMed1991

Insulin stimulates fatty acid acylation of adipocyte proteins.

Isolated rat adipocytes were incubated with [3H]palmitate in the presence or absence of insulin. Insulin caused marked (20-fold), rapid (T1/2 approximately 15 min), and concentration-dependent labeling of an 80 kilodalton protein. The insulin concentration producing half-maximal labeling was 0.4 ng/ml. The label was removed from the protein by treatment with hydroxylamine indicating the fatty acid was linked to the protein via an oxyester bond. Label removed from the protein co-eluted with palmitate standard by high-performance liquid chromatography. In summary, adipocyte proteins are subject to acylation by exogenous long-chain fatty acids and this process is stimulated by insulin.

Acylation↗
PubMed1989

Kinetics of insulin internalization and processing in adipocytes: effects of insulin concentration.

We have studied the effect of insulin concentration on the kinetics of insulin internalization and efflux in isolated rat adipocytes. To determine internalization rates adipocytes were incubated with 125I-insulin at 37 degrees C; and at frequent, early time points surface-bound and intracellular insulin were quantitated. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3.0 buffer at 4 degrees C. From this data the endocytotic (internalization) rate constant (ke) was calculated for six insulin concentrations ranging from 0.3 to 100 ng/ml. Ke was found to decrease in an insulin concentration-dependent manner (P less than .001). Thus, values for ke were 0.121 +/- 0.006 min-1 versus 0.074 +/- 0.011 min-1 at 0.3 ng/ml and 100 ng/ml, respectively. The decrease in ke did not parallel insulin concentration-dependent changes in insulin receptor affinity indicating it was not the result of an inability of low affinity receptors to be internalized. The kinetics of insulin efflux were determined by loading various concentrations of 125I-insulin into the adipocyte interior, washing away surface-bound and extracellular insulin, and then monitoring the subsequent efflux of pre-loaded insulin into medium that contained the same concentration of insulin used in the loading step. The overall rate of efflux was independent of insulin concentration. In summary, these results show that at high insulin concentrations the efficiency of insulin internalization is impaired. In contrast, the rate of insulin efflux is unaffected.

Adipose Tissue↗
PubMed1987

Effects of metalloendoprotease inhibitors on insulin binding, internalization and processing in adipocytes.

The effects of metalloendoprotease inhibitors on insulin binding, internalization, and processing were studied in isolated rat adipocytes. The metalloendoprotease inhibitor phosphoramidon caused a marked (threefold) increase in intracellular insulin accumulation without affecting surface binding. The dipeptide metalloendoprotease substrate analogues benzyloxycarbonyl-Gly-Phe-NH2 and benzyloxycarbonyl-Gly-Leu-NH2 caused similar large increases in intracellular insulin but also caused a doubling of cell surface bound insulin. The effect on surface binding was due to increased insulin receptor affinity as demonstrated by Scatchard analysis and the benzyloxycarbonyl-Gly-Phe NH2 induced inhibition of the dissociation of prebound insulin from the cell surface. These results suggest a role for endogenous metalloendoprotease-like enzymes in insulin processing by rat adipocytes.

Adipose Tissue↗
PubMedDate not supplied

The major integral membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer.

The major glycoprotein in adipocytes was purified from rat adipocyte membranes by affinity chromatography with wheat germ agglutinin-agarose followed by DEAE-Sepharose ion exchange chromatography. The protein had an apparent molecular weight of 200-kDa when analysed by SDS-PAGE under non-reducing conditions. When electroeluted from the gel, boiled in the presence of beta-mercaptoethanol, and re-analysed by gel electrophoresis, it was found to be composed of 100- and 160-kDa subunits. The N-terminal sequences were determined through 20 amino acids for each subunit, were found to be identical, and were homologous with no previously described protein sequences. The protein was not extractable from the membrane by high salt concentrations, indicating it was an integral membrane protein. Membrane fractionation by differential ultracentrifugation showed it was present predominantly in the plasma membrane fraction. The protein was susceptible to cell surface radiolabelling, further suggesting it was a plasma membrane protein. In summary, the major membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer whose disulfide-linked subunits possess identical N-terminal sequences.

Adipocytes↗