Burkitt’s Lymphoma in a 5 year old Bengali Girl: A Case Report.

Burkitt’s lymphoma (BL) is a rare monoclonal proliferation of B-lymphocytes and is classified as a poorly differentiated lymphocytic lymphoma. This tumor was first noted in Africans. The cause of this tumor is debatable, but strong evidence implicates Epstein-Barr virus in its development. This tumor predominantly affects children and is probably the fastest growing tumor in humans, with exuberant proliferation. It is a very rare malignancy accounting for only 0.76% of solid malignant tumors among Indian children. A case of BL of the right hemimandible in a 5-year-old Bengali girl is reported.

Mechanism of autoxidation of oxyhaemoglobin.

Oxyhaemoglobins from erythrocytes of different animals including fish, amphibians, reptiles, birds, mammals and human beings have been isolated by ionexchange chromatography over phosphocellulose and the comparative rates of autoxidation of oxyhaemoglobin studied. The mechanism of autoxidation in vitro has been elucidated using toad as well as human oxyhaemoglobin. Autoxidation is markedly inhibited by carbon monoxide as well as by anion ligands, namely, potassium cyanide, sodium azide and potassium thiocyanate. The inhibition by anions is in the same order as their strength as nucleophiles, indicating that it is the oxyhaemoglobin and not the ligandbound deoxy species which undergoes autoxidation. The structure of oxyhaemoglobin is considered to be mainly Hb3+O and determination of the rate of autoxidation with or without using superoxide dismutase and catalase indicates that the initial process of autoxidation takes place by dissociation of Hb3+O to methaemoglobin and superoxide to the extent of 24%. The superoxide thus produced reattacks oxyhaemoglobin to produce further methaemoglobin and hydrogen peroxide. H2O2 is a major oxidant of oxyhaemoglobin producing methaemoglobin to the extent of 53%. A tentative mechanism of autoxidation showing the sequence of reactions involving superoxide, H2O2 and OH has been presented.

Haemoglobin: A scavenger of superoxide radical.

Superoxide is continuously generated in the erythrocytes, and oxyhaemoglobin from different animals including fish, amphibians, reptiles, birds, flying mammals, mammals and human beings acts as a scavenger of superoxide. The approximate rate constants of the reaction between superoxide and oxyhaemoglobin of different animals are 0·32–1·6 × 107M–1 s–1. Results obtained with anion ligands like CN–- and N indicate that superoxide preferentially reacts with anion ligand-bound deoxyhaemoglobin. Carbonmonoxyhaemoglobin and methaemoglobin are ineffective. Work with photochemically generated oxyradical indicate that oxyhaemoglobin may also act as a scavenger of singlet oxygen. The rate constant of the reaction between superoxide and human oxyhaemoglobin is Kapp= 6·5×106 M–1 s–1, which is about three orders less than KSOD (2× 109 M–1 s–1). Thus, in the erythrocytes, oxyhaemoglobin would appear to act as a second line of defence. Oxyhaemoglobin appears to be as effective as superoxide dismutase for scavenging superoxide in the erythrocytes.

Interrelation of xanthine oxidase and dehydrogenase and L-gulonolactone oxidase in animal tissues.

There is a correlation between phylogeny and the activities of L-gulonolactone oxidase (LGO), the key enzyme responsible for ascorbic acid (AH2) synthesis in animals and total xanthine oxidase and dehydrogenase [XOD(D/O)], the enzyme responsible for the production of endogenous superoxide radical (O2-.). LGO appears in the kidneys of amphibians and reptiles but livers of mammals. XOD(D/O) also is present mainly in the kidneys of amphibians and reptiles and livers of mammals. AH2 is a potential scavenger of O2-. and it appears that tissue specific expression of LGO takes place to counteract the endogenous O2-. toxicity. The interrelation of XOD(D/O) and LGO was also observed in the liver of rats during prenatal to postnatal development.

Assay of superoxide dismutase activity in animal tissues.

Convenient assays for superoxide dismutase have necessarily been of the indirect type. It was observed that among the different methods used for the assay of superoxide dismutase in rat liver homogenate, namely the xanthine–xanthine oxidase ferricytochrome c, xanthine-xanthine oxidase nitroblue tetrazolium, and pyrogallol autoxidation methods, a modified pyrogallol autoxidation method appeared to be simple, rapid and reproducible. The xanthine-xanthine oxidase ferricytochrome c method was applicable only to dialysed crude tissue homogenates. The xanthine-xanthine oxidase nitroblue tetrazolium method, either with sodium carbonate solution, pH 10·2, or potassium phosphate buffer, pH 7·8, was not applicable to rat liver homogenate even after extensive dialysis. Using the modified pyrogallol autoxidation method, data have been obtained for superoxide dismutase activity in different tissues of rat. The effect of age, including neonatal and postnatal development on the activity, as well as activity in normal and cancerous human tissues were also studied. The pyrogallol method has also been used for the assay of iron-containing superoxide dismutase in Escherichia coli and for the identification of superoxide dismutase on polyacrylamide gels after electrophoresis.

Scavenging of superoxide radical by ascorbic acid.

Using acetaldehyde and xanthine oxidase as the source of superoxide radical, the second order rate constant for the reaction between ascorbic acid and superoxide radical was estimated to be 8·2 × 107 M-1 s–1. In rats, the average tissue concentration of ascorbic acid was of the order of 10–3 Μ and that of superoxide dismutase was of the order of 10–6 M. So, taking together both the rate constants and the tissue concentrations, the efficacy of ascorbic acid for scavenging superoxide radical in animal tissues appears to be better than that of superoxide dismutase. The significance of ascorbic acid as a scavenger of superoxide radical has been discussed from the point of view of the evolution of ascorbic acid synthesizing capacity of terrestrial vertebrates.