Effects of dithiothreitol on insulin-sensitive phosphodiesterase in rat fat cells

Effect of somatomedin C on insulin-sensitive phosphodiesterase in rat fat cells

Membrane-bound low-Km cAMP phosphodiesterase (PDE) was activated when intact rat fat cells were incubated with somatomedin C. Somatomedin C rapidly stimulated the enzyme, reaching a maximum reaction in 5 to 10 minutes. By kinetic analysis, somatomedin C activated PDE by increasing the maximal velocity (Vmax) values without altering the Michaelis-Menten constant (Km) values (0.24 +/- 0.03 mumol/L). The ED50 value of the activation by somatomedin C was very high (38.0 +/- 3.2 nmol/L) compared with that of insulin (0.22 +/- 0.07 nmol/L). This indicates that somatomedin C was about 173 times less potent than insulin in the stimulation of PDE. This potency ratio is similar to those that have been reported on lipid formation or on the other biologic insulinlike activities. When the insulin receptors were destroyed by trypsin treatment, effects of somatomedin C on the enzyme activation were abolished. This finding suggests that activation of PDE by somatomedin C was mediated through the insulin receptor.

Stimulation of low Km cyclic AMP phosphodiesterase by sulphydryl modification

Low concentrations of the organic mercurials, p-chloromercuribenzoate or p-chloromercuriphenylsulphonate activate the particulate low Km phosphodiesterase from adipose tissue and liver. Higher concentrations are inhibitory. Enzyme which has been activated by treatment of adipocytes with insulin, is not activated by the organic mercurials although inhibition by higher concentrations is seen. Enzyme from non-insulin treated adipocytes is activated and solubilised by mild trypsin treatment. Enzyme activated by either insulin treatment, or by p-chloromercuribenzoate is not further activated by trypsin, but it is solubilised.

Effects of changes in serum insulin in response to dexamethasone and adrenalectomy on insulin-sensitive phosphodiesterase in rat fat cells

The effects of dexamethasone administration and adrenalectomy on insulin-sensitive phosphodiesterase were studied in rat fat cells. Isolated fat cells were incubated at 37 degrees C for ten minutes or without insulin. A crude microsomal fraction prepared by differential centrifugation was used for the determination of phosphodiesterase level. With dexamethasone treatment (400 micrograms/kg/day) for seven days, specific activity of the enzyme and its sensitivity (ED50) to insulin were decreased, as was the maximal responsiveness to insulin. Under conditions of adrenalectomy, the specific activity and the sensitivity (ED50) were increased while the maximal responsiveness to insulin was decreased. Following dexamethasone treatment specific insulin binding was decreased, and after adrenalectomy it increased. These findings were attributed to changes in the number of insulin receptors per cell rather than to changes in affinity. Alterations in insulin sensitivity (ED50) of the enzyme seemed to be due to alterations in insulin binding to the receptor. The reduction in maximal insulin responsiveness suggested postreceptor defects in both experimental groups. The mechanism related to alterations in the specific activity was not thoroughly clarified; however, serum insulin levels may specifically affect the enzyme activity.

Activation of insulin-sensitive phosphodiesterase by lectins and insulin-dextran complex in rat fat cells

Membrane-bound low-Km cAMP phosphodiesterase was activated by concanavalin A, wheat germ agglutinin, and insulin-dextran complex under conditions of incubation with intact rat fat cells. Concanavalin A rapidly stimulated the enzyme activities and maximum was reached at 10 to 15 minutes. As little as 10 micrograms/mL concanavalin A activated the enzyme and a maximal response was obtained at 100 to 300 micrograms/mL, but concanavalin A and wheat germ agglutinin were less potent than insulin. Specific saccharide inhibitors completely abolished activation of the enzyme by lectins, but had no effect on the activation of insulin. Digestion of fat cells with 1 mg/mL trypsin for 15 minutes completely inhibited activation of the enzyme by insulin. However, concanavalin A was less sensitive to trypsinization. The insulin-dextran complex, which did not penetrate the plasma membrane, activated the enzyme and was one tenth as effective as the native insulin. These results suggest that the insulin-like actions of these lectins are provoked through coupling with the carbohydrate moiety on the cell membrane close to insulin receptors.

Comparison of phospholipid effects on insulin-sensitive low Km cyclic AMP phosphodiesterase in adipocyte plasma membranes and microsomes

Both adipocyte plasma membranes and microsomes possess insulin-sensitive low Km cyclic AMP phosphodiesterase activity. The activity of the enzyme from both sources was susceptible to activation by several anionic phospholipids. Activators of the plasma membrane enzyme were lysophosphatidylglycerol greater than lysophosphatidylcholine greater than lysophosphatidylserine greater than phosphatidylserine greater than phosphatidylglycerol. These same phospholipids activated the microsomal enzyme but the extent of activation by each phospholipid was reversed. Neutral phospholipids and other anionic phospholipids were without effect. The phospholipids had no effect on high Km cAMP phosphodiesterase in either membrane. The results suggest that the phospholipid headgroup was an important determinant for enzyme activation by phospholipid. The increased susceptibility of the plasma membrane enzyme to lysophospholipid may be attributed to a difference in the plasma membrane enzyme compared to the microsomal membrane enzyme or to differences in plasma membrane and microsomal membrane phospholipid composition and their ability to regulate low Km cAMP phosphodiesterase activity.

Insulin activates the plasma-membrane and dense-vesicle cyclic AMP phosphodiesterase in hepatocytes by distinct routes

Insulin elicits the activation of two distinct membrane-bound cyclic AMP phosphodiesterases when incubated at 37 degrees C for 5 min with intact hepatocytes: the 'dense-vesicle' enzyme and the peripheral-plasma-membrane enzyme. In hepatocytes the lysosomotropic agents chloroquine, methylamine and NH4Cl, as well as intracellular ATP depletion elicited by fructose or incubation with insulin at 22 degrees C, blocks selectively the activation of the 'dense-vesicle' enzyme. Incubation of hepatocytes with bacitracin, leupeptin and a variety of proteinase inhibitors failed to affect insulin's activation of these two cyclic AMP phosphodiesterases by distinct routes. It is suggested that activation of the 'dense-vesicle' enzyme occurs through a pathway triggered by the endocytosis, processing and recycling of the insulin receptor. This might involve the delivery, with subsequent activation, of a latent phosphodiesterase into this fraction.

Proteolytic activation and solubilization of endoplasmic-reticulum cyclic AMP phosphodiesterase activity

Dithiothreitol led to the activation and solubilization of the cyclic nucleotide phosphodiesterase activities associated with the smooth and various rough subfractions of rat liver endoplasmic reticulum. The activity in each of the subfractions exhibited somewhat different time courses, and sensitivities to dithiothreitol concentration, in respect of their solubilization and activation. Both activation and solubilization by dithiothreitol could be blocked by either thiol proteinase inhibitors or excess bovine serum albumin. Freeze-thaw solubilization was not blocked by the thiol proteinase inhibitor antipain and did not lead to the activation of the enzyme. After dithiothreitol-induced solubilization, all of the enzymes exhibited non-linear Lineweaver-Burk plots indicative of apparent negative co-operativity. In contrast, after freeze-thaw solubilization the enzyme in the smooth-endoplasmic-reticulum-plus-Golgi fraction still obeys Michaelis kinetics, as does the membrane-bound enzyme. It is possible to mimic the action of dithiothreitol in solubilizing and activating the enzyme by limited proteolysis with trypsin. Triton X-100 is highly efficient at solubilizing these enzymes, yet has little effect on their activities. Charged detergents exhibit highly selective effects on the enzymes as regards their solubilization and activity expressed.

Hypotonic activation of insulin-sensitive phosphodiesterase in rat fat cells

The effect of hypotonic treatment on the low Km membrane-bound cyclic AMP phosphodiesterase was investigated. Isolated fat cells obtained from Sprague-Dawley rats were incubated at 37 degrees C with an without 2 nM insulin. A crude microsomal fraction prepared by differential centrifugation was suspended in a hypotonic buffer at 4 degrees C, with and without protease inhibitors. Following solubilization from the particulate fraction, hypotonic treatment stimulated the phosphodiesterase in a time-dependent manner. Among the protease inhibitors, E-64, leupeptin and antipain were effective in preventing hypotonic activation of the enzyme. The release of the enzyme from the particulate fraction was partially inhibited by antipain. Kinetic analysis of the enzyme from hypotonic activation was much the same as that of the enzyme from the isotonic buffer. These results suggest that hypotonic activation of the phosphodiesterase may be the result of stimulation of an endogenous thiol protease of lysosomal origin.