LDHk, the lactate dehydrogenase associated with transformation by the Kirsten sarcoma virus: a re-evaluation

The nutrient factor queuine protects HeLa cells from hypoxic stress and improves metabolic adaptation to oxygen availability

Queuine (q), a cyclopentendiol derivative of 7-aminomethyl-7-deazaguanine, is a nutrient factor for lower and higher eukaryotes, except yeast; it is synthesized in eubacteria partly at the level of tRNA. In eukaryotes q is preferentially inserted into the wobble position of specific tRNAs in differentiated and adult tissues, but occurs mainly free in embryonic and fast proliferating cells. HeLa cells grow to a higher cell density under aerobic than under hypoxic conditions only when supplemented with q. Here we show that in hypoxically grown HeLa cells, sufficiently supplied with q, free q accumulated when serum factors become limiting while the respective tRNAs remained completely q deficient. In these cells the levels of lactate dehydrogenase A (LDH A) mRNA and of LDH A protein were at least twofold higher than in aerobically grown cells, independent of the absence or presence of q. In response to q the LDH A4 isoenzyme was further activated by a post-translational mechanism. In q-deficient HeLa cells the activity of the major anoxic stress protein, LDHk, increased as a result of hypoxia; this increase was suppressed by q. In aerobically grown, q-deficient cells significant activities of LDH A4 and LDHk were present; both activities were markedly lowered by q, while the mitochondrial electron flow was improved. The results show that free q is essential for relieving hypoxic stress in HeLa cells that results from oxygen limitation.

Lactate dehydrogenase isoenzymes

The analytical procedures for LD isoenzymes include electrophoresis, chromatography, immunochemical and kinetic methods. Electrophoretic methods are generally preferred because the resulting patterns are directly observable and all five isoenzymes are resolved in a single procedure. Chromatographic methods, with the introduction of HPLC, have recently been perfected in terms of speed, resolution, precision and accuracy. Immunochemical methods and kinetic methods are attractive because of their speed and simplicity. Therefore, the latter methods are used mainly for assaying acute myocardial infarction, where generally the determination of LD-1 and LD-2 is sufficient. In all other instances, however, electrophoretic separation is currently preferred. Ion-exchange high-performance procedures are useful prospects, particularly in view of their velocity in comparison with electrophoresis. In general, the LD isoenzymes assay contributes considerably to diagnosis, but the results must be used with an adequate knowledge of biochemistry, physiology and the advantages and drawbacks of the different assay methods used.

The major anoxic stress response protein p34 is a distinct lactate dehydrogenase

Anoxic stress is a common physiological stress, but one with unusual and significant consequences. Anoxic stress results in efficient induction of gene amplification and also plays a controlling role in the production of angiogenesis factor by macrophages. Within tumor masses, cancer cells continue to proliferate under oxygen tensions substantially lower than seen in normal tissues. The molecular basis of the anoxic stress response has not been well characterized. The major anoxic stress protein in subconfluent cell cultures is a 34-kilodalton polypeptide which has been variously reported to be either a new isozyme of lactate dehydrogenase (LDH) or the conventional muscle-type lactate dehydrogenase. This protein is of particular interest since it is also found expressed at high levels in many human cancers and has been demonstrated to be an effective serum cancer marker. We have developed an affinity chromatography procedure for purification of the anoxic stress protein p34 which effectively separates this protein from LDH-5 as well as other standard LDH isozymes. Anoxic stress protein p34 was found to specifically interact with flavins and the cellular alarmone guanosine(5')tetraphospho(5')guanosine, and also to interact with certain nucleic acids. The properties of this protein suggest that its overall role in the anoxic stress response may be in the coordination of a number of cellular systems.

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human LDH-A, skeletal muscle isoenzyme

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human skeletal muscle isoenzyme, since the partial amino acid sequences of human liver LDH-K/A protein were found to be identical with the known primary structure of human LDH-A isoenzyme and the LDH-A isoenzymes from human placenta and bovine muscle were shown to be tyrosylphosphorylated. This tyrosylphosphorylated LDH-K/A protein was also found to be complexed with 21 kD, 30 kD, and 56 kD proteins.

Nucleotide sequences of the cDNA and an intronless pseudogene for human lactate dehydrogenase-A isozyme

Eight cDNA clones for lactate dehydrogenase-A isozyme (LDH-A) were isolated from a human fibroblast cDNA library, characterized, and no sequence heterogeneity was found. Four cDNA clones appear to contain nearly full-length cDNA inserts and the complete nucleotide sequence of 1710 base pairs consists of the protein-coding sequence (999 base pairs), the 5' (97 base pairs) and 3' (565 base pairs) untranslated regions and poly(dA) tail (49 base pairs). The predicted amino acid sequence of the human LDH-A polypeptide shows 92% homology (27 differences out of 331 amino acids compared) with that of the pig LDH-A subunit determined by direct protein sequencing [Kiltz et al. (1977) Hoppe-Seyler's Z. Physiol. Chem. 358, 123-127]. Human genomic clones containing an LDH-A pseudogene were isolated and the nucleotide sequence of 1635 base pairs from an intronless pseudogene was determined. The presence of two termination codons, two deletions of three nucleotides each and the replacement of three arginine residues at the active site (nos 98, 105 and 168) by other amino acids renders its coding region incapable of producing a functional LDH-A protein. A comparison between human LDH-A cDNA and the pseudogene sequences reveals 12.9% differences (114 transitions, 65 transversions and 36 deletions/insertions). Further, only four out of the 25 dCpdG dinucleotides present in the cDNA sequence remain unchanged, although the sequences possess 87.1% homology.

On the mechanism of production of superoxide radical by reaction mixtures containing NADH, phenazine methosulfate, and nitroblue tetrazolium

In aerobic reaction mixtures containing NADH, phenazine methosulfate, and nitroblue tetrazolium, O2- production is mediated by the tetrazolium, not the phenazine. Thus, superoxide dismutase inhibited reduction of the tetrazolium, but when ferricytochrome c was substituted for the tetrazolium its reduction was not affected by this enzyme. Furthermore, NADH plus the phenazine did not accelerate the oxidation of epinephrine to adrenochrome unless the tetrazolium was present, and under those circumstances superoxide dismutase did inhibit adrenochrome formation. When the tetrazolium and ferricytochrome c were present simultaneously, addition of superoxide dismutase was seen to accelerate the reduction of the cytochrome. This is explainable by the reduction of O2- by the reduced phenazine, which thus competes with cytochrome c for the available O2-. When the O2- was eliminated by superoxide dismutase, more of the reduced phenazine was available for the direct reduction of cytochrome c.

A cancer-associated lactate dehydrogenase is expressed in normal retina

An unusual isozyme of lactate dehydrogenase, lactate dehydrogenase k, is found in high concentrations in cultured cells transformed by the Kirsten murine sarcoma virus and in many human cancer tissues. In experiments described here high levels of a lactate dehydrogenase k activity were detected in extracts of normal rodent retina. This activity had the same key properties as the human tumor isozyme, namely, a highly cathodic electrophoretic mobility and inhibition of enzymatic activity by oxygen and 5',5'-dipurinenucleoside tetraphosphates. Expression of this activity in the retina may be related to the high aerobic glycolysis characteristic of the retina, a metabolic feature shared with many tumors.