Analysis of human hypoxanthine-guanine phosphoribosyl transferase isozymes by isoelectric focusing in polyacrylamide gel

The method of isoelectric focusing in polyacrylamide gel was used to separate HGPRT isoenzymes in crude hemolysates of human and rat erythrocytes. HGPRT from erythrocytes of a normal human male donor consistently revealed three peaks of activity. Their mean isoelectric points, using pH 5–7 range ampholytes, were, peak I, pI 6.00; peak II, pI 5.8³; and peak III, pI 5.71. Peak III was wide and tailed. It always had a shoulder with a mean pI of 5.62. HGPRT from rat erythrocytes revealed two peaks of activity, corresponding to isoelectric points of 5.90 and 5.80. The method of isoelectric focusing in polyacrylamide gel is presented as a new way of detecting isoenzyme patterns.This study was supported by Grant No. 38-5768 from the Lebanese National Council for Scientific Research and Grant No. 18-5240 from the Medical Research Committee, American University of Beirut.     

The separation of adenine and hypoxanthine-guanine phosphoribosyl transferases isoenzymes by disc gel electrophoresis

Hypoxanthine-guanine (HGPRT; E.C. 2.4.2.8) and adenine (APRT; E.C. 2.4.2.7) phosphoribosyl transferases were studied by disc electrophoresis on polyacrylamide gel. The positions of the isoenzymes were detected by radiochemical enzyme assay. The nucleotide products of the reactions were precipitated in the gel with lanthanum chloride. APRT was found to migrate slightly less rapidly than albumin and produced a single narrow symmetrical peak of activity. HGPRT migrated 25–50% more slowly than albumin and produced a broad zone of activity consisting of four unequal peaks. The APRT enzyme of Rhesus monkey liver and the HGPRT enzyme of sheep erythrocytes migrated notably slower than the corresponding human enzymes. An isoenzyme of APRT was detected in human erythrocytes which migrated more rapidly than that of most individuals. In all instances, the adenine was utilized by one electrophoretic component and hypoxanthine and guanine by another. Furthermore, the components which utilized hypoxanthine and guanine were inseparable. The sensitivity of the assay made it possible to assess the electrophoretic and enzymatic characteristics of HGPRT isoenzymes on aliquots of hemolysates capable of producing 0.5 picomoles of IMP per minute. In human erythrocytes with normal enzyme content, this amount of activity is present in approximately 50 nanoliters of cells.Aided by U.S. Public Health Service grants Nos. HD 04608 and HD 03015 from the National Institute of Child Health and Human Development, National Institutes of Health.     

Radioimmunofixation of human ferritin following serum isoelectric focusing

Radioimmunofixation of human ferritin following isoelectric focusing of serum was developed to study the microheterogeneity of this protein in native serum without previous purification or concentration. This method requires only 2-10 microliter of serum and can be used with levels of ferritin as low as 10 micrograms/l. In this way, the extensive microheterogeneity of this protein was revealed, since in some cases it produced as many as 35 bands with isoelectric points in a pH range of 4.95-5.9. Very different isoelectric focusing patterns (spectrotypes) of ferritin were observed during the investigation of pathological sera. The high sensitivity of this technique makes it useful for the investigation of serum ferritin in diseases involving modifications of the metabolism of this protein.     

High-resolution one-dimensional isoelectric focusing of mouse MHC class I antigens

A method is described for a biochemical comparison of mouse class I antigens utilizing antisera with a monomorphic pattern of reaction and high-resolution one-dimensional isoelectric focusing (1D-IEF). The most commonly occurring and studied H-2K and D alleles were identified in a comparison of over 40 mouse strains. By comparing H-2 mutant mouse strains, cell lines transfected with defined class I genes, or mice transgenic for a mouse class I gene and H-2 recombinant mouse strains, unambiguous identification of class I alleles was possible. The complex pattern presented by H-2-heterozygous mice was readily resolved into the contribution by the individual parental alleles. The H-2 ^b bm series of mutants was analyzed, and for those mutants where a charge difference was predicted based on their known sequence, a change in isoelectric point (IEP) was indeed observed. Based on analysis by IEF, the bm8 mutant may contain (an) amino acid substitution(s) in addition to those reported. The present method further appears useful in elucidating defects in class I antigen synthesis and post-translational modifications, as these cannot be easily characterized when using surface staining with monoclonal antibodies (mAbs).     

Methods of isoelectric focusing

The method of isoelectric focusing has been avoided by many workers because of expense, technical difficulty, and problems of interpretation. Inexpensive, easy, and interpretable results are possible using equipment and reagents commonly available. Methods which allow these results are presented and explained.     

Solubilization,stabilization and isoelectric focusing of human liver neuraminidase activity

Homogenate preparations of human liver have been prepared and over 75% of the particulate neuraminidase activity (which comprises approx. 90% of the total activity) has been solubilized using 0.85% (w/v) Triton X-100 in 25 mM phosphate buffer (pH 6.8). The solubilized neuraminidase activity is extremely labile, but can be stabilized for at least 4 weeks at 2–4°C, using 10 mM N-acetylneuraminic acid. Kinetic characterization of homogenate and solubilized supernatant fluid neuraminidase activities indicated comparable pH optimum curves (maximum activity at pH 4.5–4.7) and apparent K_m values (0.2–0.4 mM) for the synthetic fluorometric substrate $\text{4-}\text{methylbelliferyl}\text{-α-}\text{D}\text{-N-}\text{acetylneuraminic}$ acid. Isoelectric focusing has been performed on human liver homogenates and Triton X-100-solubilized neuraminidase activities, and the presence of several forms (4–6) with isoelectric points (pI values) between 4.4 and 5.2 has been demonstrated in both preparations. The similar kinetic and isoelectric focusing properties of the two preparations suggest that the solubilized enzyme activity is representative of the homogenate activity and that the solubilized enzyme is suitable for purification purposes.     

Heterogeneity and specificity of human anti-insulin antibodies determined by isoelectric focusing

Anti-insulin antibodies are present in the majority of insulin treated diabetics, and in some cases these antibodies have been found to be highly specific for limited epitopes on the molecule. To determine how the human response differs from that seen in inbred animals, we have examined the heterogeneity and specificity of human anti-insulin antibodies by isoelectric focusing (IEF). In addition, we have used human insulin to examine the extent of autoreactivity in the serum of subjects treated with animal insulins. The majority of diabetic sera exhibited complex IEF spectra that were composed of discrete bands and unresolved smears. Autoradiography using 125I-beef, pork, and human insulin revealed some affinity differences; however, the predominant antibodies were capable of binding all insulins, including human. These specificity studies were extended by comparing competitive inhibition with excess cold insulins, and sera with highly specific binding of the A chain loop of beef insulin were identified. The spectra by IEF of these highly specific sera were found to be variable. Our results indicate that the majority of insulin-treated diabetics develop a heterogeneous antibody response that is more complex than the response of inbred animals and includes reactivity with autologous insulin. Although infrequent, individuals having antibodies directed at limited regions of the molecule can be identified and will provide valuable tools for dissecting this complex response.     

Influence of fatty acid and time of focusing on the isoelectric focusing of human plasma albumin

In the isoelectric focusing of human plasma albumin, two major peaks of pI 4.8 and 5.6 are observed. As fatty acids are removed from the albumin either by defatting before the focusing experiment or gradually during the focusing experiment, the pI 4.8 peak diminishes and the pI 5.6 peak increases. The interpretation of this effect is confirmed by fatty acid analysis before and after focusing. Alkylation of the free sulfhydryl group changes the focusing position of the defatted peak to pI 5.7. Otherwise the isoelectric focusing pattern of human albumin is unaffected by the status of the free sulfhydryl group or by the ampholine concentration.     

Microheterogeneity of human kininogen by isoelectric focusing and crossed immunoelectrophoresis.

LMW kininogen was isolated from whole human plasma by gel filtration on Sephadex G-200 (Kav 0.34) followed by DEAE-chromatography according to earlier established methods. Further purification was performed with specific Sepharose-antibody columns to remove protein contaminants, avoiding procedures which may denature kininogen. The microheterogeneity was investigated by isoelectric focusing in column in the pH-gradients 3.5-10, 4-6 and 3.5-5. Kininogen components were determined by single radial immunodiffusion against monospecific anti-human kininogen serum, in comparison with focusing of whole plasma. 40% of isolated as well as whole plasma kininogen focused at pI 4.5; the respective focusing ranges were pI 4.4-4.7 (60--80%) and pI 4.3-4.6 (92%). The results were verified by crossed immunoelectrophoresis. The pI 4.5 component is apparently the main native form of human kininogen as shown by focusing of whole human blood bank plasma. Earlier described difficulty of separating kininogen and alpha2HS-glycoprotein was verified by crossed immunoelectrophoresis which showed approximately seven kininogen components after focusing in polyacrylamide gel electrophoresis at pI 4.5-5.0 and four alpha 2HS components at pI 4.2-4.6.     

Reversion in expression of hypoxanthine-guanine phosphoribosyl transferase following cell hybridization.

Hybridization of mutant cell lines deficient in hypoxanthine-guanine phosphoribosyl transferase (HGPRT; E.C.: 2.4.2.8) from a variety of established rodent sources with HGPRT plus human cells yielded progeny cells which grew in selective medium containing hypoxanthine, aminopterin and thymidine (HAT). The same result was obtained when the human cell used was an HGPRT minus transformed line derived from a patient with the Lesch-Nyhan syndrome. Electrophoretic analysis indicated that all HAT-resistant progeny clones contained an active HGPRT enzyme which was indistinguishable from the wild type enzyme of the corresponding normal rodent cells. In contrast, no HAT-resistant cells have been obtained when the same HGPRT minus rodent cells were subjected to fusion processes in the absence of human cells or when they fused with similarly derived HGPRT minus mutant cells of other rodents. Reversion in expression of the rodent gene for HGPRT was detected in clones which retained one or more human chromosomes and in clones which contained no detectable human chromosomal material. The observed re-expression of rodent HGPRT in HAT-resistant clones suggests that HGPRT plus as well as HGPRT minus human cells contributed a factor which determined the expression of respective rodent structural genes for HGPRT. In contrast, HGPRT minus rodent cells were unable to induce the synthesis or normal HGPRT in the cells derived from the patient with the Lesch-Nyhan syndrome.