An affinity-purified, monospecific rabbit antibody against soluble human milk galactosyltransferase was used to localize the enzyme in HeLa cells by immunofluorescence and by the protein A-gold technique at the electron microscope level. Specific immunofluorescence was observed in a juxtanuclear cytoplasmic region which was identified, on immunostained thin sections of low-temperature Lowicryl K4M-embedded HeLa cells, as Golgi apparatus. Label by gold particles was limited to two to three trans cisternae of the Golgi apparatus, indicating a compartmentalization of galactosyltransferase in the cisternal stack. Combination of preembedding thiamine pyrophosphatase cytochemistry, with postembedding immunostaining for galactosyltransferase proved codistribution of the two enzymes. However, the acid phosphatase-positive, trans-most cisterna was negative for galactosyltransferase. The close topological association of both galactosyltransferase and thiamine pyrophosphatase (or nucleoside diphosphatase) suggests a concerted action of both enzymes in glycosylation.

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