Experiments investigating the intraorganelle availability of nucleotide sugars have shown that UDP-Xyl and UDP-Glc
can also be found in the ER, whereas UDP-GlcA and UDP-Glc
can be found in the Golgi (38), suggesting that the corresponding NSTs are yet to be identified.
Linear regression analysis of measured amounts (y; corrected for endogenous concentrations) vs added amounts (x) gave the following coefficients (standard errors in parentheses): UDP-Gal, y = 0.92 ([+ or -] 0.001)x + 0.006 ([+ or -] 0.002); UDP-Glc, y = 0.98 ([+ or -] 0.004)x - 0.02 ([+ or -] 0.011); UDP-GalNAc, y = 0.97 ([+ or -] 0.001)x + 0.005 ([+ or -] 0.002); and UDP-G1cNAc, y = 1.02 ([+ or -] 0.001)x - 0.002 ([+ or -] 0.002); [R.sup.2] >0.999 for all.
Considering the amounts of UDP-sugars found in the pooled muscle tissues, a minimum of 37, 13, 32, and 20 mg of tissue (wet weight) would be sufficient to accurately quantify UDP-Gal, UDP-Glc, UDP-GalNAc, and UDP-G1cNAc, respectively.