Studies on the regulation of the three enzymes of the Leloir pathway in cultured mammalian cells. I. Effect of substitution of galactose for glucose as the sole hexose in the medium in human diploid cell strains and in a rat hepatoma line

Metabolic effects of galactose on human HepG2 hepatoblastoma cells

HepG2 cells were used as a model system to study the effects of galactose overload on the liver, a target organ of galactose toxicity in patients suffering from transferase-deficient galactosemia. In the presence of galactose, HepG2 cell growth was slow and the pattern of gene expression remained characteristic of liver cells (secretion of alpha-fetoprotein [AFP] albumin, and transferrin). Galactose-1-phosphate (Gal-1-P) accumulated, as it does in galactosemic cells, but did not affect the energetic status of the cells (no adenosine triphosphate [ATP] depletion). However, the substitution of galactose for glucose as the sole hexose in the medium affected the specific activities of the galactose-metabolizing enzymes. Galactokinase (GALK) activity was decreased, and those of galactose-1-phosphate uridyltransferase (GALT), phosphoglucomutase, and glucose-6-phosphate dehydrogenase (G6PDH) were increased. The conversion of radiolabeled galactose to glucose (CO2 production and glycogen level) was greater in galactose medium than in glucose medium after a 7-day culture. Therefore, the culture of HepG2 cells in galactose medium indicates that the enhanced utilization of this hexose is due to the increased enzyme activities regulating its own metabolism. Hence, HepG2 cells constitute a good model for the study of modulation of galactose-metabolizing enzymes by galactose.

Impaired hexose uptake by diploid skin fibroblasts from galactosaemic patients. Connection with cell growth and amino acid metabolism, and possible bearing on late-onset clinical symptoms

In skin fibroblasts of patients presenting with galactosaemia, either from galactose 1-phosphate uridyltransferase or galactokinase deficiency, a deficit in extracellular glucose utilization was observed. This deficit was constant over 3 weeks of continuous cell growth in a medium containing 5.5 mmol/L glucose as the only hexose, and homologous serum. Levels of glucose utilization by deficient skin fibroblasts were stable at about 65-70% of the glucose utilization of control normal skin fibroblasts. Cell morphology was normal, and cell growth was subnormal during this period. However, the energy provision appeared sufficient for cellular needs since cell growth in this glucose medium was observed not to depend on the presence of extracellular glutamine. In contrast, glutamine was required for growth of galactosaemic fibroblasts cultured in medium containing 5.5 mmol/L galactose. If expressed in many cell types, this impaired glucose uptake would be expected seriously to damage highly glucose-dependent tissues such as the central nervous system. This might be of relevance to the persistent neurological damage observed in many galactosaemic patients in spite of their compliance with an early strict galactose-free diet.

Galactose and cataract

Galactosemia is a disorder caused by a deficiency of any one of three possible enzymes involved in the metabolism of galactose: galactokinase, transferase or epimerase. Any single deficient enzyme can result in cataract through the accumulation of galactitol in the lens. The ophthalmologist may play an important role in this disease, since early recognition of cataract development followed by the initiation of a galactose-free diet may lead to clearing of lenticular opacities. The clinical and laboratory findings that distinguish the three enzyme deficiency disorders of galactosemia are discussed. The biochemical genetics of each enzyme also are reviewed, along with the recent evidence linking heterozygous galactokinase deficiency and presenile cataract.

Immunological and genetic characterization of 2-deoxygalactose-resistant, galactokinase-deficient mutants of Chinese hamster cells: evidence for structural mutations at the galK locus

Ten independent mutants resistant to 2-deoxygalactose and without any detectable galactokinase activity (null-galactokinase mutations) were isolated from mutagenized Chinese hamster somatic cells. They were analyzed for the presence of serologically cross-reacting material (CRM) with antiserum generated against highly purified Chinese hamster galactokinase. All 10 mutants contain cross-reacting material (i.e., were CRM+), indicating that all the mutations affect the correct expression of a product of the galactokinase structural gene. Complementation analysis among them shows that the 10 mutations fall in one functional genetic unit.

Evidence for a striking increase in acetylcholinesterase activity in cultured human fibroblasts which are trisomic for chromosome two

Acetylcholinesterase (AchE) is reported to have a narrowly restricted distribution among human tissues. Three strains of human fibroblasts which are trisomic for chromosome 2 had an average level of AchE activity over 28 times higher than the average level in 19 control strains of human fibroblasts. In contrast, the mean pseudocholinesterase activity of the trisomy-2 strains did not differ appreciably, or significantly, from the mean for the control strains. The 19 control strains included 10 strains trisomic for autosomes other than 2, and 9 euploid strains. Our estimate of the mean AchE activity in the control strains did not differ significantly from zero and might, in any case, have originated from a minute amount of activity contributed to the cells by an esterase in our culture medium. Despite the striking elevation of AchE activity in fibroblasts trisomic for chromosome 2, extracts of these cells had only about 1.5% of the specific AchE activity (per microgram DNA) present in extracts of human cerebral cortex. None of the 22 strains studied had detectable activity for two other enzymes which, like AchE, have a restricted distribution among human tissues: xanthine oxidase and choline acetyltransferase.

Studies on the regulation of the three enzymes of the Leloir pathway in cultured mammalian cells. II. A search for quantitative interrelationships between the three enzyme activities

Studies on a normal human diploid cell strain revealed that the specific activity of the cell protein, for each of the three enzymes of the Leloir pathway, changed significantly as the cells grew. The kinetics of change in specific activity varied according to the enzyme being studied, and the kinetics for each enzyme varied from experiment to experiment. Within each experiment, there was no consistent correlation between specific activity for any one enzyme and specific activity for the other two. The ratios between the specific activities did not tend to remain constant as the absolute levels of specific activity changed. Hence, the activities did not behave coordinately. The kinetics of change in these ratios varied from experiment to experiment. The failure of galactose to stimulate increased cellular activity for the three enzymes (shown in the preceding paper), and the absence of a coordinate relationship between the activities, represent a striking difference between the behavior of these enzymes in human diploid cell strains and their behavior in E. coli.