HUMAN FIBROBLASTS PREFER MANNOSE OVER GLUCOSE AS A SOURCE OF MANNOSE FOR N-GLYCOSYLATION - EVIDENCE FOR THE FUNCTIONAL IMPORTANCE OF TRANSPORTED MANNOSE
K. Panneerselvam et al., HUMAN FIBROBLASTS PREFER MANNOSE OVER GLUCOSE AS A SOURCE OF MANNOSE FOR N-GLYCOSYLATION - EVIDENCE FOR THE FUNCTIONAL IMPORTANCE OF TRANSPORTED MANNOSE, The Journal of biological chemistry, 272(37), 1997, pp. 23123-23129
Mannose in N-linked oligosaccharides is assumed to be derived primaril
y from glucose through phosphomannose isomerase (PMI), The discovery o
f mammalian mannose-specific transporters that function at physio logi
cal concentrations suggested that mannose might directly contribute to
oligosaccharide synthesis, To determine the relative contribution of
glucose and mannose, human fibroblasts were labeled with either [2-H-3
]mannose or [1,5,6-H-3]glucose at the same specific activity, and the
N-linked chains were released by PN-Gase F digestion, Most of the tric
hloroacetic acid-precipitable [H-3]mannose label was released by this
digestion, but only about 10% of the trichloroacetic acid precipitable
material was released from cells labeled with [1,5,6-H-3]glucose. Bot
h sugars labeled a similar array of oligosaccharides, and acid hydroly
sis of these chains showed that [2-H-3]mannose contributed 65-75% of t
he [H-3]mannose in cells labeled for 1 h, despite the 100-fold higher
concentration of exogenous glucose, Mannose consumption and [2-H-3]man
nose utilization were within the range of rates expected for mannose t
ransport via the mannose-specific transporter. About 7-14% of the [2-H
-3]mannose is used for glycosylation, while the rest (86-93%) is catab
olized to (H2O)-H-3 via PMI, Increasing the exogenous mannose concentr
ation beyond mannose transporter saturation results in the conversion
of >99% of [2-H-3]mannose into (H2O)-H-3. Long term labeling of cells
with [2-H-3]mannose showed that the specific activity of mannose in gl
ycoproteins reached 77% of the specific activity of [2-H-3]mannose add
ed to the medium, These results show that when fibroblasts are provide
d with physiological concentrations of mannose, they use the mannose-s
pecific transporter to supply the majority of mannose needed for glyco
protein synthesis, PMI may normally be used to catabolize excess manno
se rather than to primarily supply Man-6-P for glycoprotein synthesis.