Human platelets contain functional surface and intracellular glycosyltransferases. (A) GalNAc-transferase, galactosyltransferase, and sialyltransferase activities in platelets. Detergent-extracted platelets were incubated with UDP-¹⁴C-GalNAc, UDP-¹⁴C-Gal, or CMP-¹⁴C-sialic acid (SA) and acceptor substrates. See supplemental Methods for details on acceptor substrates. Shown are the means ± SD of 3 experiments. (B) Immunoblots of platelet lysates subjected to SDS-PAGE using mAbs specific for GalNAc-T1, GalNAc-T2, GalNAc-T3, β4Gal-T1, and ST3Gal-I glycosyltransferases. Asterisks indicate bands with molecular weights corresponding to the glycosyltransferases (n = 3). (C) Flow cytometry histograms of GalNAc-T1 (red), β4Gal-T1 (green), and ST3Gal-I (blue) expression in nonpermeabilized (solid line) and permeabilized (dotted line) resting platelets. Isotype control mAbs (black line; n = 3). (D) Galactosyltransferase and Golgi marker (GM130) distribution in resting platelets determined using anti-GM130 and anti–GalNAc-T1, β4Gal-T1, and ST3Gal-I mAbs (n > 5). Insets show platelets at a higher magnification. Scale bars indicate 5 μm.