Determination of [U-13C]glucose turnover into various metabolite pools for the differential diagnosis of lactic acidemias

Stable-isotope tracer experiments performed in vitro are evaluated for their utility in differentiating between pyruvate dehydrogenase and cytochrome oxidase deficiencies, two of several enzyme defects commonly associated with the lactic acidemias. Fibroblasts of enzyme-deficient individuals and of age-matched controls are grown in medium containing [U-13C]glucose. Direct analysis of cells and conditioned culture medium provides only minor differences in the organic acid/amino acid GC-MS profiles, making differentiation of enzyme defects difficult by this method. However, differences have been found in the glucose turnover into various cell metabolites, making differentiation of these two enzyme defects possible. The cellular pool of glutamic acid experiences 13C-enrichment in both the control and cytochrome oxidase deficient lines, but not in the pyruvate dehydrogenase-deficient line. The cellular pool of an unknown, possibly an aminopentose sugar, on the other hand, experiences 13C-enrichment in the pyruvate dehydrogenase and control lines, but not in the cytochrome oxidase line. These observations, as well as other differences in the extent of enrichment into various metabolite pools, suggest that this stable-isotope approach, in vitro, is feasible and may allow these two enzyme defects to be differentiated in a definitive manner. Such stable-isotope experiments are easy to carry out with cultured cells and are inexpensive. Applications of the technique to other genetic disorders might be appropriate.     

Growth control of human foreskin fibroblasts and inhibition of extracellular sialidase activity by 2-deoxy-2,3-dehydro-N-acetylneuraminic acid

Normal human fibroblasts have two extreme modes of existence in culture, quiescent and proliferative. The growth and division of these cells are usually well regulated by the action of various endogenously generated stimulators and inhibitors. We have speculated that an extracellular sialidase may be involved in the regulation of growth and that inhibition of this activity might decrease or abolish cell growth. To test this hypothesis, we have incubated preconfluent cultures of fibroblasts in the presence and absence of a potent sialidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid. Treatment of cells with this inhibitor resulted in the inhibition of an extracellular sialidase activity for up to 24 h and had a marked growth inhibitory effect in a concentration-dependent manner. The effect of the inhibitor on cell proliferation was specific and reversible. During a chase period of 48 h after pulse labeling cells with [3H]N-acetylmannosamine and [14C]serine, there was a 15% decrease of [3H]sialic acid in the membrane-bound GM3 with 80 microM inhibitor in the medium, as compared with a 32% decrease in the controls. Our results suggest that an extracellular sialidase may participate in cell-surface modifications that accompany (or control) the changes observed when cells traverse the cell cycle, from the quiescent to the proliferative phase.     

Methods for computing the standard errors of branching points in an evolutionary tree and their application to molecular data from humans and apes

Statistical methods for computing the standard errors of the branching points of an evolutionary tree are developed. These methods are for the unweighted pair-group method-determined (UPGMA) trees reconstructed from molecular data such as amino acid sequences, nucleotide sequences, restriction-sites data, and electrophoretic distances. They were applied to data for the human, chimpanzee, gorilla, orangutan, and gibbon species. Among the four different sets of data used, DNA sequences for an 895-nucleotide segment of mitochondrial DNA (Brown et al. 1982) gave the most reliable tree, whereas electrophoretic data (Bruce and Ayala 1979) gave the least reliable one. The DNA sequence data suggested that the chimpanzee is the closest and that the gorilla is the next closest to the human species. The orangutan and gibbon are more distantly related to man than is the gorilla. This topology of the tree is in agreement with that for the tree obtained from chromosomal studies and DNA-hybridization experiments. However, the difference between the branching point for the human and the chimpanzee species and that for the gorilla species and the human-chimpanzee group is not statistically significant. In addition to this analysis, various factors that affect the accuracy of an estimated tree are discussed.      

Secretary sialidase activity and GM3 ganglioside

The sialidase activities with GM3 ganglioside and sialyllactitol were demonstrated in the conditioned medium of human fibroblasts. pH versus activity profiles of conditioned medium with GM3 as substrate suggested the presence of two sialidases with optimal activities at pH 4.5 and pH 6.5. The GM3 sialidase activity at pH 6.5 was suppressed in the medium of contact-inhibited cells. This sialidase may function in the metabolism of cell surface GM3 since there was a selective loss of labeled sialic acid from GM3 at different times of incubation after pulse-labeling with a radioactive sialic acid precursor ([3H]N-acetyl-mannosamine) and a radioactive ceramide precursor ([14C]serine). In addition, a sialidase inhibitor, 2-deoxy-2, 3-dehydro-N-acetyl-neuraminic acid (NeuAc-2-en) resulted in a reversible growth inhibitory effect and the suppression of the sialidase activity in the medium. We have speculated that GM3 hydrolysis on the cell surface by the sialidase may be coordinated with the cell cycle and may be at its maximum during early in the G1 phase.     

Aberrant fatty acyl alpha-hydroxylation in human neuroblastoma tumor gangliosides

Abnormalities of ganglioside structure characterize the neoplastic state, and aberrant glycosylation has been implicated as underlying many new tumor ganglioside structures. However, variations in ceramide structure can also result in novel tumor gangliosides. To address systematically this aspect of ganglioside metabolism, we have initiated a study of the structures of the ceramide species of an oligosaccharide-homogeneous human tumor-derived ganglioside, GM2. The ganglioside was isolated from neuroblastoma tissue and purified by normal-phase high pressure liquid chromatography. Marked ceramide heterogeneity was observed; 18 individual ceramide species of neuroblastoma GM2 were separated by reversed-phase high pressure liquid chromatography and collected. Their structures were determined by a combination of negative- and positive-ion fast atom bombardment mass spectrometry and collisionally activated dissociation tandem mass spectrometry of the underivatized gangliosides. The striking finding was the detection of alpha-hydroxylation of a significant fraction of each of the major fatty acid species (16:0, 18:0, 20:0, 22:0, and 24:1); alpha-hydroxylated species quantitatively represented almost one-fifth of the total tumor GM2 species. Fatty acyl hydroxylation was also detected in the ceramide of several other human tumor gangliosides. In contrast, as previously known, fatty acyl hydroxylation was not detected in the normal human brain gangliosides GM3, GM2, and GM1. We propose that aberrant fatty acid alpha-hydroxylation is a novel and sometimes quantitatively significant characteristic of human tumor ganglioside metabolism.     

Mitogenic effects of bacterial neuroaminidase and lactosylceramide on human cultured fibroblasts.

Exogenously added bacterial neuraminidase and lactosylceramide both stimulated the growth of cultured human skin fibroblasts. Neuraminidase (100 units/ml) increased DNA synthesis 1.9-fold and cell density 1.4-fold after 24 and 48 h, respectively, in culture. Treated fibroblasts contained less ganglioside NeuAc alpha 2-3Gal beta 1-4GlcCer (GM3), presumably due to neuraminidase-catalyzed hydrolysis to lactosylceramide. Addition of lactosylceramide (100 microM) to the fibroblast culture medium also increased DNA synthesis threefold within 24 h and cell density twofold after 48 h. These findings are compatible with a mechanism by which the proliferation of human fibroblasts is regulated by the relative levels of GM3 and lactosylceramide in the plasma membrane.