Enzyme polymorphisms in man

There are a large number of different enzymes synthesized in the human organism, and many of these probably contain more than one structurally distinct polypeptide chain. If current theories about genes and proteins are correct we must suppose that the primary structure of each of these different polypeptides is determined by a separate gene locus, and that there are probably also other loci which are specifically concerned with regulating the rate of synthesis of particular polypeptides or groups of polypeptides. Furthermore, we may expect that genetical diversity in a human population will to a considerable extent be reflected in enzymic diversity. That is to say, in differences between individuals either in the qualitative characteristics of the enzymes they synthesize, or in differences in rates of synthesis. The work I am going to discuss was largely aimed at trying to get some idea of the extent and character of such genetically determined enzyme diversity among what may be regarded as normal individuals. When my colleagues and I started on this line of work about three years ago the information available about this aspect of the subject was very limited. It had of course been recognized for quite a long time that there are many rare metabolic disorders, the so-called ‘inborn errors of metabolism’, which are due to genetically determined deficiencies of specific enzymes (Harris 1963). These conditions can in general be attributed to mutant genes which result either in the synthesis of an abnormal enzyme protein with defective catalytic properties, or in a gross reduction in rate of synthesis of a specific enzyme protein. By and large such genes appear to be relatively uncommon and have frequencies of between 0·01 and 0·001 in the general population. Heterozygotes often show a partial enzyme deficiency though they are usually in other respects quite healthy. A few cases are also known where a specific enzyme deficiency occurs quite commonly in certain populations. The most extensively studied example of this is glucose 6-phosphate dehydrogenase deficiency, and it seems likely that in this particular case the relatively high incidence in certain populations is attributable to a specific selective advantage which the deficiency may confer in situations where endemic malaria is an important selective agent (Motulsky 1964).

The human alkaline phosphatases: what we know and what we don't know

A review of the human alkaline phosphatases dealing specifically with (1) the gene loci, (2) characterization and discrimination of the various enzymes, (3) polymorphism at the enzyme level, (4) cDNA and gene structures, (5) membrane binding, (6) the carbohydrate moieties, (7) hypophosphatasia, (8) alkaline phosphatases in malignancies, (9) function.

Isolation and characterization of a cDNA encoding a human liver/bone/kidney-type alkaline phosphatase

Alkaline phosphatases (ALPs) [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] isolated from human liver, bone, and kidney (L/B/K) exhibit very similar biochemical and immunologic properties that differentiate them from other human ALPs, such as those characteristically found in placenta and intestine. Despite their similarities, the L/B/K ALPs produced in different tissues show slight physical differences. To examine structural and evolutionary relationships between the various ALPs, a cDNA corresponding to L/B/K ALP mRNA has been isolated. A lambda 11 cDNA expression library was constructed using poly(A) RNA from the osteosarcoma cell line Saos-2 and screened with anti-liver ALP antiserum. The 2553-base-pair cDNA contains an open reading frame that encodes a 524 amino acid polypeptide with a predicted molecular mass of 57.2 kDa. This ALP precursor protein contains a presumed signal peptide of 17 amino acids followed by 37 amino acids that are identical to the amino-terminal sequence determined from purified liver ALP. In addition, amino acid sequences of several CNBr peptides obtained from liver ALP are found within the cDNA-encoded protein. The deduced L/B/K ALP precursor polypeptide shows 52% homology to human placental ALP and 25% homology to Escherichia coli ALP precursor polypeptides. Sixty percent nucleotide homology exists between the human L/B/K and placental cDNAs over the protein coding regions. The 5' and 3' untranslated regions of the L/B/K ALP cDNA, 176 and 805 base pairs, respectively, show no homology to the corresponding regions of placental ALP cDNA.

A missense mutation in the human liver/bone/kidney alkaline phosphatase gene causing a lethal form of hypophosphatasia

Hypophosphatasia is an inherited disorder characterized by defective bone mineralization and a deficiency of serum and tissue liver/bone/kidney alkaline phosphatase (L/B/K ALP) activity. Clinical severity is variable, ranging from death in utero (due to severe rickets) to pathologic fractures first presenting in adult life. Affected siblings, however, are phenotypically similar. Severe forms of the disease are inherited in an autosomal recessive fashion; heterozygotes often show reduced serum ALP activity. The specific gene defects in hypophosphatasia are unknown but are thought to occur either at the L/B/K ALP locus or within another gene that regulates L/B/K ALP expression. We used the polymerase chain reaction to examine L/B/K ALP cDNA from a patient with a perinatal (lethal) form of the disease. We observed a guanine-to-adenine transition in nucleotide 711 of the cDNA that converts alanine-162 of the mature enzyme to threonine. The affected individual, whose parents are second cousins, is homozygous for the mutant allele. Introduction of this mutation into an otherwise normal cDNA by site-directed mutagenesis abolishes the expression of active enzyme, demonstrating that a defect in the L/B/K ALP gene results in hypophosphatasia and that the enzyme is, therefore, essential for normal skeletal mineralization.

New assays for the tyrosine hydroxylase and dopa oxidase activities of tyrosinase

New assays for the tyrosine hydroxylase and dopa oxidase activities of tyrosinase (EC 1.14.18.1) have been developed. The tyrosine hydroxylase assay uses L-[carboxy-14C]tyrosine as the substrate, 14CO2 is released from the products of the hydroxylation and further metabolism of L-[carboxy-14C]tyrosine by incubation with ferricyanide, and measured radiometrically. D-Dopa is a preferable cofactor to L-dopa for the assay. Dopa oxidase activity is measured spectrophotometrically. Dopaquinone, produced on the oxidation of L-dopa, reacts with Besthorn's hydrazone (3-methyl-2-benzothiazolinone hydrazone) to form a pink pigment with an absorbance maximum at 505 nm. Details of the optimisation of conditions for the assays and their specificities for the two enzyme activities are described.

Artificial heterokaryons of animal cells from different species

A virus, inactivated by ultraviolet light, was used to fuse together cells from different species of vertebrate, and the resulting heterokaryons were examined by autoradiographic and cytological techniques. Heterokaryons could be made with both differentiated and undifferentiated cells: HeLa and Ehrlich ascites cells were studied as examples of undifferentiated cells; rabbit macrophages, rat lymphocytes and hen erythrocytes as examples of differentiated cells. These last three cells were chosen because in them, in varying degrees, the process of differentiation has resulted in suppression of the synthesis of DNA or of both DNA and RNA. This suppression was in all cases found to be reversible: the dormant nuclei could be induced to resume the synthesis of RNA or DNA or both when the differentiated cells were fused with a cell which normally synthesizes RNA and DNA. Observations on heterokaryons in which differentiated cells were fused with HeLa cells and with each other permitted certain general conclusions to be drawn about the regulation of nucleic acid synthesis in the heterokaryon. It was found that if either one of the parent cells normally synthesized RNA, RNA synthesis took place in both types of nuclei in the heterokaryon. If either of the parent cells normally synthesized DNA, DNA synthesis took place in both types of nuclei in the heterokaryon. If neither of the parent cells synthesized DNA, no DNA synthesis took place in the heterokaryon. In all cases where a cell which synthesized a particular nucleic acid was fused with one which did not, the active cell initiated the synthesis of this nucleic acid in the inactive partner. In no case did the inactive cell suppress synthesis in the active partner.

The nuclei of heterokaryons in which DNA synthesis took place underwent mitosis, and those nuclei which entered mitosis synchronously usually fused together. This process resulted in the progressive formation of mononucleate hybrid cells, which might thus contain within a single nucleus chromosomal complements derived from different species. These mononucleate hybrid cells were also capable of RNA and DNA synthesis, and many of them in turn underwent mitosis. At metaphase these cells showed, in various combinations, the chromosomal complements of the two parent cells. Mononucleate hybrid cells formed by the fusion of a large number of single cells did not appear to be capable of continued multiplication; but mononucleate cells containing one chromosomal set from each parent cell were still found to be undergoing mitosis many days after cell fusion.

The reactivation of the red cell nucleus

When the nucleus of a mature hen erythrocyte is introduced into the cytoplasm of a HeLa cell it resumes the synthesis of RNA and DNA. This reactivation of the red cell nucleus in the heterokaryon is associated with a marked increase in its volume. There is a direct relationship between the volume of the nucleus and the amount of RNA which it makes. The nuclear enlargement is not a consequence of increased RNA synthesis, or of DNA synthesis: enlargement is the primary event, and the increase in RNA synthesis is determined by it. The possibility is considered that changes in nuclear volume may regulate not only the amount of RNA made in the nucleus but also the areas of chromatin on which it is made.

Identification of PTSD in cancer survivors

The authors measured the rate and determinants of posttraumatic stress disorder (PTSD) in a group of cancer survivors. Patients who had a history of cancer diagnosis with at least 3 years since diagnosis, receiving no active treatment, such as chemotherapy or radiation, were interviewed (N = 27). Patients, who were part of the DSM-IV PTSD field trial, were compared with a community-based control group matched for age and socioeconomic status. One member of the survivor group (4%) and no members of the control group met criteria for current PTSD (NS). Six of the survivors (22%) and no control subjects met lifetime criteria (P < 0.02). Cancer patients have a higher rate of PTSD than found in the community. Symptoms closely resemble those of individuals who have experienced other traumatic events.

A new marker for human cancer cells. 1 The Ca antigen and the Ca1 antibody

In a search for antibodies that might distinguish between malignant and non-malignant cells a panel of matched pairs of hybrid cells produced by fusion of diploid fibroblasts with malignant cells originating from a cervical carcinoma was used as a screen. Each pair consisted of a hybrid in which malignancy was suppressed and a malignant segregant derived from this hybrid. A monoclonal antibody, designated Ca1, was found that discriminated absolutely between the hybrids in which malignancy was suppressed and the malignant segregants. This antibody detected an antigen present in the cell membranes of a wide variety of malignant human cells lines but not of diploid human cell strains. The antigen was found in very low concentrations, if at all, in homogenates of normal adult or fetal tissues. It could be immunoprecipitated by the Ca1 antibody from extracts of malignant cells but not from extracts of non-malignant cells. After reduction, the immunoprecipitated antigen separated in sodium dodecyl sulphate acrylamide gels as two bands with proximate molecular masses of 390 000 and 350 000. These two components had a properties of glycoproteins with a high carbohydrate content; both bound the Ca1 antibody. The antigenic determinant resisted boiling at 100 degrees C and extraction by range of organic solvents. The binding of the Ca1 antibody to the antigen was substantially reduced by treatment of the antigen with neuraminidase, and the antigenic determinant was largely destroyed by certain endoglycosidases and by extensive proteolysis. Pending its further characterisation, this antigen had been called the Ca antigen.

Expression of a human placental alkaline phosphatase gene in transfected cells: use as a reporter for studies of gene expression

The human placental alkaline phosphatase gene has been cloned and reintroduced into mammalian cells. When a plasmid carrying the gene under control of the simian virus 40 early promoter (pSV2Apap) is transfected into a variety of different cell types, placental alkaline phosphatase activity can readily be detected by using whole cell suspensions or cell lysates. Alkaline phosphatase activity can also be visualized directly in individual transfected cells by histochemical staining. The gene is appropriate for use as a reporter in studies of gene regulation since its expression is dependent on the presence of exogenous transcription control elements. The overall assay to detect the expression of the gene is quantitative, very rapid, and inexpensive. Cotransfections of cells with pSV2Apap and a related plasmid carrying the bacterial chloramphenicol acetyltransferase gene (pSV2Acat) indicate that transcription of these two genes is detected with roughly the same sensitivity.

Differential inhibition of the products of the human alkaline phosphatase loci

1. Inhibition studies have been carried out on a series of ALPs derived from liver, bone, kidney, placenta and intestine, using L-phenylalanine, L-homoarginine, L-leucine, L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine as inhibitors. 2. No differences between liver, bone and kidney ALPs with any of the inhibitors were observed. 3. L-phenylalanine and L-homoarginine give a major degree of discrimination between liver/bone/kidney ALP on the one hand, and placental and intestinal ALPs on the other. L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine give a major degree of discrimination between placental ALP on the one hand, and intestinal ALP and liver/bone/kidney ALP on the other. L-leucine discriminates between the three classes, but to a lesser degree. Minor degrees of discrimination between placental and intestinal ALPs occur with L-phenylalanine and L-homoarginine and between intestinal and liver/bone/kidney ALPs with L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine. By using an appropriate combination of inhibitors the ALPs can be separated into three clearly distinct categories: placental, intestinal and liver/bone/kidney. 4. The six common placental ALP phenotypes as defined by electrophoresis show identical inhibition profiles with the series of inhibitors. The same profile was found for several rare electrophoretic variants. However, two rare electrophoretic variants (P-187 and P-92) each encountered once in a sample of 225 plancentae, showed significantly deviant inhibitions with the various inhibitors and also differed from each other. From the electrophoretic patterns, both of these rare phenotypes appear to be heterozygotes. P-187 probably corresponds to the so-called D-variant previously described. P-92 represents a new type of placental ALP variant with an aberrant inhibition profile. In both cases the particular rare allele concerned evidently alters the primary structure of the enzyme so that it has an altered electrophoretic mobility and also an altered sensitivity to inhibition for each of the different inhibitors. 5. Treatment of the various ALPs with neuraminidase to remove sialic acid residues does not affect their inhibition characteristics or activities.

The expression of genetic information: a study with hybrid animal cells

When the nucleus of a hen erythrocyte is introduced into the cytoplasm of a human or mouse cell in culture, it resumes the synthesis of RNA. The reactivated erythrocyte nucleus undergoes great enlargement, but it does not, for at least 2 or 3 days, develop nucleoli which can be discerned under the light microscope. During this period, the heterokaryon, although it may contain several active erythrocyte nuclei, does not synthesize any hen-specific surface antigens; and the hen-specific antigens introduced into the surface of the heterokaryon by the process of cell fusion are eliminated. But when, later, the erythrocyte nuclei do develop nucleoli, hen-specific antigens reappear on the surface of the heterokaryon and progressively accumulate.

Before developing nucleoli, the erythrocyte nuclei synthesize little, if any, normal 28 S or 16 S RNA; but they do synthesize large amounts of the RNA which shows polydisperse sedimentation in conventional sucrose density gradients. Autoradiographic studies involving the use of a microbeam of ultraviolet light show, however, that this ‘polydisperse’ RNA is not transferred to the cytoplasm of the cell in detectable amounts so long as the erythrocyte nucleus lacks a definitive nucleolus. The inability of the erythrocyte nucleus at this stage to determine the synthesis of hen-specific surface antigens is thus attributable to the fact that it fails to transfer the RNA made on its chromosomes to the cytoplasm of the cell. When the erythrocyte nuclei develop nucleoli, however, the RNA which they make is transferred to the cytoplasm of the cell, and the synthesis of hen-specific surface antigens then begins. These experiments suggest that the nucleolus may play a decisive role in the transfer of information from nucleus to cytoplasm. The possible nature of this role is discussed.