Biological potency of covalently linked insulin-receptor in rat adipocytes. Comparison with the potency of reversible complexes

Recognition of covalent insulin-receptor complexes on viable adipocytes by anti-insulin antibodies

Isolated rat adipocytes were photo-affinity-labelled with B2-(4-azido-2-nitrophenylacetyl)-des-PheB1-insulin or B29-(4-azido-2-nitrophenylacetyl)insulin. Four anti-insulin antibodies (3 monoclonal, 1 polyclonal) were tested for their ability to inhibit the persistent stimulation of lipogenesis caused by the covalently bound insulin [Brandenburg et al. (1980) Nature (London) 286, 821-822]. The polyclonal and 2 monoclonal antibodies, directed against the C-terminus of the B-chain, gave a significant depression, while one antibody, directed against the region A(8-10), was without effect. Under reversible conditions, without irradiation, all antibodies completely inhibited lipogenesis. For the polyclonal antibody this is shown in a dose-dependent way. It is concluded that the effective antibodies can recognize their epitope because it is accessible on the surface of the complex and does not represent part of the receptor-binding surface of insulin. This binding leads to interference with the generation and/or transmittance of the biological signal.

Activation-dependent redistribution of cellular components alters susceptibility of human neutrophils to cross-linking agents

Contrary to resting cells, neutrophils stimulated with concanavalin A resist inhibition by bifunctional N-hydroxysuccinimide esters. Con A prestimulated, cross-linker-treated cells released superoxide upon restimulation with PMA but did not respond to chemotactic peptides. Although rates of PMA-elicited NADPH oxidase activity were lowered by the treatment, the activation parameters, namely lag times of the reaction, were not altered. The protection by Con A against blockade by cross-linkers developed concomitantly to the activation of NADPH oxidase and indicated redistribution of cross-linker-susceptible cellular components responsible for activation with PMA. The identity of these components is discussed.

Time-dependence of biological activity induced by covalent insulin-receptor complexes in rat adipocytes

Lipogenesis in isolated adipocyte preparations is stimulated when photosensitive insulin derivatives are attached covalently to specific receptors. This response was compared quantitatively with that to reversibly associated insulin, and it was shown that both covalent and reversible insulin-receptor complexes behave very similarly. The extent of stimulation of lipogenesis was studied as a function of time. Cells were incubated in buffer for various times before addition to vials containing 0 (basal) or 10 ng of monocomponent insulin/ml (maximal) and [U-3H]glucose. After 60 min, the toluene-soluble [3H]lipids were measured. The maximal stimulation induced by reversibly bound insulin was virtually constant over a period of 4 h. In contrast, adipocytes to which N alpha B2-(2-nitro-4-azidophenylacetyl)-des-PheB1-insulin had been covalently attached at the start of the experiment showed a loss of stimulation with time when incubated at 37 degrees C. This loss was decreased in the presence of lysosomotropic agents such as chloroquine at concentrations (approx. 200 microM) that had very little or no effect on the basal and maximal lipogenesis rates. A simple method was used to transform the measured rate of loss of stimulation into a rate of loss of effective units. A half-time of 80 min was calculated for the effective covalent insulin-receptor units in adipocytes at 37 degrees C at pH 7.4. This is very close to values reported by others for the internalization of covalent complexes in these cells, suggesting that this may be the causative event for the deactivation of the insulin-receptor unit. The inhibitory effect of chloroquine on the deactivation may indicate that the insulin-receptor complex can function even after internalization.

Biological action and fate of photoaffinity-labelled insulin-receptor complexes

Covalent linking of two photoactivatable insulin derivatives, B2-(2-nitro,4-azidophenylacetyl)-des-PheB1-insulin and B29-(2-nitro,4-azidophenylacetyl)-insulin to viable rat adipocytes gives a system, which contains a fixed stoichiometry between hormone and receptor. The biological signal of prolonged lipogenesis has been used to study several aspects of insulin binding and action: the role of the site of the crosslink between insulin and receptor, recognition of bound photoinsulin by anti-insulin antibodies, the half-life of the biologically active complex, the pH-dependence of the biological signal, and the possible role of internalization. Furthermore, the effect of trypsin on the insulin receptor, as well as the insulin-receptor complex, has been investigated and a refined model of the receptor is presented.

The biological potency of covalent insulin-receptor complexes. Dependence on site of cross-linkage

A radioactive photosensitive insulin analogue, 125I-N epsilon B29-(4-azido-2-nitrophenyl-acetyl)insulin, was covalently bound to the receptors of isolated rat adipocytes by irradiation with UV light. This caused a stimulation of lipogenesis. The relative potency of the covalent complexes to that of normal reversible complexes was calculated by comparing the amounts of radioactivity required to be covalently or reversibly bound by adipocytes to cause the same levels of stimulation. For several different occupancies , this relative potency was constant at 50 +/- 3%. Previous studies had shown that the relative potency of covalently bound 125I-N alpha B2-(4-azido-2- nitrophenylacetyl )des- PheB1 -insulin was only 25 +/- 4% under identical conditions. This demonstrates that the sites of crosslinking have a marked effect on the potency of the covalent hormone-receptor complex. It appears that attachment through the C-terminus of the B-chain leads to a better stabilization of the biologically active form than linking through the more flexible N-terminus.

Biological activity of covalently-linked insulin-receptor complexes in rat adipocytes. Effect of pH

The relative molar potencies of covalently and reversibly-bound insulin-receptor complexes were studied as a function of pH. The insulin derivatives used were 125I-B2 (2-nitro-4-azidophenylacetyl)des-PheB1-insulin and 125I-B29(2-nitro-4-azidophenylacetyl)des-PheB1-insulin. The potencies of both types of reversible complexes were effectively identical and constant between pH 7 and 8. The relative potency of the covalent B2-complex increased from 25 to 75%, and of the covalent B29 complex from 30 to nearly 100%. This indicates that the covalently linked partners in the complex are able to flex about the cross-linkages. Variations in the potency are due to variations in the number of correctly associated, reversibly or covalently bound insulin-receptor complexes. The form of the pH dependance suggests that an ionizable group, possibly an amino group, must be deprotonated to allow effective interaction.