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Enzymatic digests of proteins isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) were separated by capillary high-performance liquid chromatography (HPLC). The column eluate was split to an electrospray mass spectrometer on one side and to both a UV detector and a microblotter on the other side. Using the microblotter, the peptides eluted from the column were collected directly onto a polyvinylidene difluoride (PVDF) membrane for Edman sequencing. Thus, a peptide mass map from the mass spectrometric analysis and a prepared PVDF membrane for subsequent Edman sequencing were generated in a single experiment. The addition of molecular mass information to the blotted LC eluate is useful for determining the most important peaks to undergo Edman sequencing. Coupling the capillary HPLC with a microblotter to electrospray mass spectrometry provides an integrated system for separation, collection, and structural analysis of protein digests. It provides high levels of sensitivity, recovery, and convenience for protein characterization. Proteins loaded onto SDS-PAGE at low picomole levels can be analyzed by the new integrated system.

The primary structure and posttranslational modifications of rabbit lens alpha-crystallins were examined using electrospray ionization mass spectrometry to determine the molecular weights of the intact proteins and fast atom bombardment mass spectrometry to analyze proteolytic digests of the alpha A- and alpha B-crystallins. The previously determined primary structure of alpha A-crystallin was confirmed. Posttranslational modifications detected included one phosphorylation site and the presence of a truncated form minus the five C-terminal residues. The previously undetermined amino acid sequence of rabbit alpha B-crystallin was determined to be the same as the bovine alpha B-crystallin sequence except at three residues: Thr 40, Thr 132, and Pro 153. Rabbit alpha B-crystallin showed evidence of phosphorylation at the same three sites as bovine alpha B-crystallin. The molecular weights of the intact proteins indicated that any one molecule had a maximum of two phosphorylations. Also, there was a truncated form which did not include the five C-terminal residues.

alpha-Crystallins from the water-soluble and the water-insoluble, guanidine-soluble portions of lenses from four renal failure patients and two normal donors of similar age were isolated and enzymatically digested into peptides. Molecular weights of the peptides, determined by fast atom bombardment mass spectrometry, indicated modifications specifically associated with renal failure. The only modifications observed in the alpha-crystallins from renal failure patients, but not in the normal old lenses, were glutathione adducts to Cys 131 and Cys 142. These adducts were present in the lenses of all four renal failure patients, but not in the two normal old lenses. The four lenses from the renal failure patients were searched for evidence of carbamylation at lysyl or cysteinyl residues: carbamylation was not detected. Because the same mass spectrometric methods had previously demonstrated sufficient sensitivity and specificity to detect as little as 5% modification in the examination of in vitro carbamylated bovine lenses, these results indicated that carbamylation is not a major modification of the lens alpha-crystallins of renal failure patients.

A new procedure to selectively identify disulfide-containing peptides in extracts of biological tissues is described. Disulfide-containing peptides are detected by their UV absorbance and electrochemical (EC) activity after chromatographic separation, and subsequently identified by fast atom bombardment mass spectrometry (FABMS). This combination of fractionation by HPLC and selective detection is attractive because it is rapid, highly specific for disulfide-containing peptides, and applicable to all disulfide-containing peptides that may be present in complex biological mixtures. Useful procedures for applying the method are demonstrated with tissue extracts from bovine pituitary and catfish pancreas. In addition to finding the expected disulfide-containing peptides, evidence for two forms of catfish insulin are presented. The merits of this and other methods used to detect peptides in similar tissue extracts are discussed.