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Effect of Valine on Propionate Metabolism in Control and Hyperglycinemic Fibroblasts and in Rat Liver

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Effect of Valine on Propionate Metabolism in Control and Hyperglycinemic Fibroblasts and in Rat Liver Pediat. Res. 11: 749-753 (1977) Fibroblasts propionyl-CoA carboxylase methylmalonic acidemia rat liver methylmalonyl-CoA mutase valine nonketotic hyperglycinemia Effect of Valine on Propionate Metabolism in Control and Hyperglycinemic Fibroblasts and in Rat Liver 36 BETTY REVSIN,' ' JOYCE LEBOWITZ, AND GRANT MORROW Ill Department of Pediatrics, University of Arizona College of Medicine, Arizona Medical Center, Tucson, Arizona, USA Summary propionic acidemia (3), and isovaleric _acidemia (2), w?ereas others exhibit no ketosis, e.g., nonketot1c hyperglycmemrn (4). Measurement of methylmalonyl-CoA mutase and propionyl­ Those hyperglycinemic states associated with ketosis appear !o CoA carboxylase activities in lysates from fibroblasts derived be diseases which result from primary enzyme defects either IO from control, nonketotic hyperglycinemia, propionic acidemia, the propionate pathway (3, 21) or in one of the catabolic path­ and both vitamin B 12-responsive and -nonresponsive variants of ways of branched chain amino metabolism (2, 14). As a result, methylmalonic acidemia showed only one abnormality: a 59% the hyperglycinemia appears to be a secondary phenomena. In decrease in carboxylase activity in the nonketotic hyperglyci­ nonketotic hyperglycinemia (NKH), it has been suggested that nemic lysates (P < 0.01). When fibroblasts from all cell types the primary defect causes hyperglycinemia and_ results from were grown on valine-supplemented (24 mM) media, m?t~se decreased activity of the glycine cleavage reaction (26, 34). activity was generally inhibited. As for carboxylase achv~ty, However, recent studies have demonstrated diminished glycine control lines were inhibited 35% as compared to controls with­ cleavage activity in the ketotic forms of hyperglycinemia as ~ell out valine and propionic acidemia activity was undetectable. On as in the nonketotic form (25, 32). These latter observations the other hand, carboxylase activity in both methylmalonic aci­ suggest that the decreased glycine cleavage activity found in the demia variants was increased 40% and nonketotic hyperglycine­ NKH syndrome may be a secondary phenomena possibly due to mia carboxylase activity was increased 80% (P < 0.01) when mechanisms similar to those seen in the ketotic states. grown on valine-supplemented media. Isoleucine could not sub­ Studies from our own laboratory have demonstrated that fi­ stitute for valine in producing increased carboxylase activity in broblasts derived from patients with NKH have a decreased these mutants. ability to transport glycine across the cell membrane (29). The Glycine cleavage activity in fresh rat liver homogenates (11.1 K for the controls and the NKH cell lines were similar indicat­ µmol/mg protein/90 min) did not vary significantly when 24 mM ini that the affinity of the respective carrier protein(s) for glycine valine was added to the reaction (9.9 µmol/mg protein/90 min). was no different in the mutant lines. However, the V max was Therefore, the hyperglycinemia observed in both ketotic and significantly lower in the NKH lines as compared to the controls, nonketotic forms is probably not caused by a direct effect of indicating that the movement of the carrier across the membrane valine on the glycine cleavage reaction. and/or dissociation of the carrier and the glycine molecule are These data suggest that the presence of increased amounts of depressed (24, 29). propionic acid in serum or urine does not necessarily rule out the A report presented several years ago suggested that IO acute possibility of nonketotic hyperglycinemia due to the decreased NKH, propionate metabolism was normal (8). In a more re­ activity of the carboxylase enzyme. cently published report, the same author hypothesized that the "true nonketotic hyperglycinemic state" is one in which both Speculation propionate and branched chain amino acid metabolism are nor­ Decreased activity of the propionyl-CoA carboxylase enzyme mal in vitro and in vivo (7). Furthermore, the report stated that NKH in nonketotic hyperglycinemia fibroblasts suggests altered propi­ oral loading of patients with leucine, isoleucine, valine, methionine and threonine had no clinical effect on the patient onate metabolism in this mutation. Additionally, growth of non­ (7). These' data are incompatible with several other reports ketotic hyperglycinemia fibroblasts on high concentrations of valine results in greatly increased carboxylase activity. This ef­ describing coma in NKH patients after oral valine loads ( 16, 1 7, 20). Our NKH patient became comatose upon oral administra­ fect of valine is also evident in methylmalonic acidemia lysates tion of valine. Since isoleucine and leucine were without effect, although no decrease in carboxylase activity in these muta?ts grown under normal conditions is observed. Although the valme the coma appeared to be valine specific (20). The reasons for the different chemical responses to vahne effect is not presently understood, it may be possible that nonke­ totic hyperglycinemia can be diagnosed from fibroblasts grown require further investigation in order to identify t~e underly!ng defect or defects in NKH. Indeed, the mechanism by which in tissue culture based on the combination of these two findings: hyperglycinemia manifests itself as a characteristic of the ketotic (J) decreased propionyl-CoA carboxylase activity in nonk~t?tic hyperglycinemic diseases also remains unknown. hyperglycinemic lysates grown under normal cult_u~e ~ond1hons In contrast to the report of normal propionate metabolism in and (2) increased propionyl-CoA carboxylase actmty m nonke­ nonketotic hyperglycinemia (7, 8), evidence derived from stud­ totic hyperglycinemic lysates grown on valine-supplemented me­ ies in our laboratory have shown altered propionyl-CoA carbox­ dia. ylase activity in fibroblasts derived from NKH patients and from some methylmalonic acidemia (MM-emia) fibroblasts when Hyperglycinemia is associated with a variety ~f dis_ease~. Some grown on valine-supplemented media. The present manuscript are associated with ketosis such as methylmalomc ac1demrn (21), presents further in vitro data concerning the activity of propio- 749 750 REVSIN, LEBOWITZ, AND MORROW III nate enzymes (propionyl-CoA carboxylase and methylmalonyl­ METHYLMALONYL-CoA MUTASE ACTIVITY (TABLE 2) CoA mutase), as well as the effect of the branched chain amino 14 In our MM-cmia lines in~orporation ~f CO2 into methyl­ acid, valine, on these activities in control and mutant fibroblasts. 14 malonate from NaH CO3 1s an expression of propionyl-CoA carboxylase and methylmalonyl-CoA racemase activities due to MATERIALS AND METHODS the fact that the mutasc enzyme is defective in the MM-cmia lines. Thus the 14 C accumulates as methylmalonate. However, in FIBROBLASTS the control and NKH fibroblasts the majority of the 14C label Fibro~lasts from control, NKH, propionic acidemia (PA), and accumulates as succinyl-CoA since all of the propionate pathway MM-em1a explants were grown on Eagle's minimum essential enzymes are_ theoretically intact. Little metabolism of succinyl­ media supplemented with 10-12 % fetal calf serum, 100 units/ml CoA occurs m these cultured cells because of greatly diminished penicillin, and 100 µg/ml streptomycin. Media was changed succinic dehydrogenase activity (35). Nonetheless, small every third day. All cells were below passage 20. amounts of label can be recovered from methylmalonyl-CoA, For the enzyme assays, fibroblasts were routinely grown to tricarboxylic acid cycle intermediates, and as bicarbonate. It was confluence in disposable roller bottles (Belko), harvested with important, therefore, to measure methylmalonyl-CoA mutase activity in these cell lines to determine whether the decreased 0.125% trypsin-Versene, and centrifuged at 500 x g for 5 min. The cells were washed once with 12 ml saline/roller bottle of carboxylase activity of the NKH cells was indeed due to de­ cells and centrifuged again at 500 x g for 5 min. The washed creased activity at that step or whether the decreased activity was fibroblasts were resuspended in water (0 .1 ml cells/0 .15 ml a reflection of impaired mutase function. water) and lysed by freeze-thawing five times in a dry ice­ Table 2 shows that the mutase activity in the control, NKH, acetone ~ath. The resulting lysate was centrifuged at 2000 x g PA, and vitamin B1rrcsponder lysates is quite similar. The for 10 mm. The supernatant was used in all following enzyme nonresponder mutase activity is almost nil since it is the apoen­ assays. zyme that is defective in this mutation (19). As a result of the Mc_thylmalo~yl-CoA carbonylmutase (mutasc) was assayed as similar mutasc activities in these control and mutant cell lines, ~c~cnbed previously (22), and propionyl-CoA carboxylasc ac­ the depressed carboxylase activity observed in the NKH lines is t1v1ty (carboxylase) was measured according to the method of probably due to an effect at that step in propionate metabolism Giorgio and Whitaker (13) with some modifications (22). rather than at the mutase level. When cells were grown on increased concentrations of valine or isoleucine, the roller bottles were seeded in the usual manner EFFECT OF VALINE ON CARBOXYLASE AND MUTASE ACTIVITIES (TABLES I AND 2) and allowed to grow for 3 days. At that time, the media contain­ ing the increased concentration of valine were introduced to the Since valine is toxic to both NKH and MM-cmia patients, it cells. Growth of the cells was allowed to continue until conflu­ was of interest to determine
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