No abstract

The purification of proteins by conventional procedures is frequently laborious and incomplete, and the yields are often low. Enzyme isolation based on a highly specific biological property-strong reversible association with specific substrates or inhibitors-has received only limited attention.1-4In affinity chromatography, the enzyme to be purified is passed through a column containing a cross-linked polymer or gel to which a specific competitive inhibitor of the enzyme has been covalently attached. All proteins without substantial affinity for the bound inhibitor will pass directly through the column, whereas one that recognizes the inhibitor will be retarded in proportion to its affinity constant. Elution of the bound enzyme is readily achieved by changing such parameters as salt concentration or pH, or by addition of a competitive inhibitor in solution.The successful application of the method requires that the adsorbent have a number of favorable characteristics. Thus, the unsubstituted matrix or gel should show minimal interaction with proteins in general, both before and after coupling to the specific binding group. It must form a loose, porous network that permits easy entry and exit of macromolecules and which retains favorable flow properties during use. The chemical structure of the supporting material must permit the convenient and extensive attachment of the specific ligand under relatively mild conditions, and through chemical bonds that are stable to the conditions of adsorption and elution. Finally, the inhibitor groups critical in the interaction must be sufficiently distant from the solid matrix to minimize steric interference with the binding processes.In this report the general principles and potential application of affinity chromatography are illustrated by results of its application to the purification of staphylococcal nuclease, a-chymotrypsin, and carboxypeptidase A. The solid matrix used in these studies was Sepharose (a "beaded" form of the cross-linked dextran of highly porous structure, agarose5) which displays virtually all the desirable features listed above. Activation of the Sepharose by treatment with cyanogen bromide6 7 results in a derivative that can be readily coupled to unlrotonated amino allg. The resultant Sepharose-inhibitor gel is a highly stable structure which has nearly ideal properties for selective column chromatography.Experimental Procedure.-Materials: Sepharose 4B was obtained from Pharmacia, cyanogen bromide from Eastman, pdTp and benzoyl-L-tyrosine ethyl ester from Calbiochem.Staphylococcal nuclease (Foggi strain) was obtained by modifications8 of techniques described by Fuchs et al.9 The following purified enzymes were purchased from Worthington: a-chymotrypsin (CDS 7LC); a-chymotrypsin, DFP-treated (CD-DIP 204); chymotrypsinogen A (CGC 8CC); subtilisin VIII (66B 3080); trypsin, DFP-treated 636

Insulin stimulates the activity of mitogen-activated protein kinase (MAPK) via its upstream activator, MAPK kinase (MEK), a dual specificity kinase that phosphorylates MAPK on threonine and tyrosine. The potential role of MAPK activation in insulin action was investigated with the specific MEK inhibitor PD98059. Insulin stimulation of MAPK activity in 3T3-L1 adipocytes (2.7-fold) and L6 myotubes (1.4-fold) was completely abolished by pretreatment of cells with the MEK inhibitor, as was the phosphorylation of MAPK and pp90Rsk, and the transcriptional activation of c-fos. Insulin receptor autophosphorylation on tyrosine residues and activation of phosphatidylinositol 3'-kinase were unaffected. Pretreatment of cells with PD98059 had no effect on basal and insulin-stimulated glucose uptake, lipogenesis, and glycogen synthesis. Glycogen synthase activity in extracts from 3T3-L1 adipocytes and L6 myotubes was increased 3-fold and 1.7-fold, respectively, by insulin. Pretreatment with 10 microM PD98059 was without effect. Similarly, the 2-fold activation of protein phosphatase 1 by insulin was insensitive to PD98059. These results indicate that stimulation of the MAPK pathway by insulin is not required for many of the metabolic activities of the hormone in cultured fat and muscle cells.

No abstract