Fig. 1.

Oc from a single human blood plasma sample analyzed by MALDI-MSIA. The intact, full length protein is the most abundant form, but several previously undocumented N- and C-terminally truncated forms are also evident (Table I). Evidence described in the text indicates that these did not appear to form artifactually ex vivo. Because of prompt fragmentation induced by the laser (21–23), γ-carboxyl post-translational modifications could not be observed by MALDI-MS. The spectrum shown is a single representative of spectra from 130 specimens analyzed (supplemental Fig. S5). Lower mass shoulder peaks as observed on peaks A, D, and F correspond to in-source neutral loss of water or ammonia. Higher mass shoulder peaks correspond to sodium adducts [M + Na]⁺.