Tumor pretargeting with biotinylated antibody/avidin complexes improves the therapeutic index of systemically administered biotin-tumor necrosis factor (TNF) conjugates. Since the number of biotins in this conjugate is known to be critical for activity, we have characterized the structure of different biotin-TNF conjugates, prepared by reaction with D-biotinyl-6-aminocaproic acid N-hydroxysuccinimide ester and identified the biotinylation sites by trypsin digestion, reverse-phase chromatography, and electrospray mass spectrometry analyses. The results have shown that N-terminal valine is a preferential biotinylation site at pH 5.8, half of biotins being located on the alpha -amino group of this residue in a conjugate bearing one biotin/trimer (on average). Moreover, evidence has been obtained to suggest that the remaining part of biotins are linked to the E-amino group of lysine 128, 112, and 65, while lysine 11, 90, and 98 were practically unmodified. No evidence of O-biotinylation of serine, threonine and tyrosine was obtained. (C) 2001 Academic Press.
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