Synthesis and characterization of carbohydrate-insulin conjugates using desorption ionization mass spectrometry

Solid-phase derivatization of tryptic peptides for rapid protein identification by matrix-assisted laser desorption/ionization mass spectrometry

Solid-phase sulfonation of tryptic peptides adsorbed to C18 muZipTips has been carried out to facilitate de novo sequencing with mass spectrometry. Peptides are reacted with the sulfonation reagent while they are still adsorbed to the solid phase. Excess reagent passes through the ZipTip to waste. Washing the products before subsequent elution from the mini-column also affords sample cleanup prior to analysis. Near quantitative N-terminal sulfonation can be achieved reliably at room temperature in only a few seconds. The method has been applied successfully to model peptides and to solution or in-gel digests of proteins. Current sequencing limits are about 100 fmol of protein. Multiplexed sample sulfonation reactions have been carried out with a manual 8-position micropipettor or using centrifugal force to reliably pass reagents and wash solutions over sample-loaded ZipTips. With multiplexing, overall preparation times have been reduced to about 1 min per sample. The solid-phase format facilitates efficient use of precious digest samples by enabling them to be recovered from the matrix-assisted laser desorption/ionization (MALDI) sample stage after mass fingerprinting, derivatized and re-analyzed by MALDI postsource decay mass spectrometry.

Synthesis and characterization of insulin-fluorescein derivatives for bioanalytical applications

Human insulin was labeled with fluorescein isothiocyanate (FITC) and fully characterized to yield four distinct insulin-FITC species. High-performance liquid chromatography and electrospray mass spectrometry were used to determine the extent and location of fluorescein conjugation. By changing the reaction conditions (i.e., pH, time, and FITC/insulin ratio) the selectivity of the fluorescein conjugation was altered, and all conjugates could be separated. The isolated species of insulin-FITC were labeled at the following residues: A1(Gly), B1(Phe), A1(Gly)B1(Phe), and A1(Gly)B1(Phe)B29(Lys). All four insulin-FITC conjugates were then used to develop fluorescence polarization binding assays with monoclonal and polyclonal anti-insulin antibodies. The assay sensitivity differed between the conjugates depending on the site of modification (B1 > A1 > A1B1 > A1B1B29). Also, the type of antibody used had an important role in the binding of insulin-FITC conjugates. Finally, for the first time the biological activity of the four conjugates was demonstrated by an autophosphorylation assay. The positional substitution dramatically affected the biological activity, confirming insights into the residues responsible for the insulin binding region. The B1 conjugate was found to retain almost all biological activity while the A1 and A1B1 conjugates had approximately 10 times lower activity. The trisubstituted species (labeled at A1, B1, and B29) was determined to be least active.