B cell chronic lymphocytic leukemia (B-CLL) is a neoplastic disorder characterized by accumulation of B lymphocytes due to uncontrolled growth and resistance to apoptosis. Analysis of B cells freshly isolated from 40 patients with chronic lymphocytic leukemia demonstrated that the Src kinase Lyn, the switch molecule that couples the B cell receptor to downstream signaling, displays anomalous properties. Lyn is remarkably overexpressed at the protein level in leukemic cells as compared with normal B lymphocytes, with a substantial aliquot of the kinase anomalously present in the cytosol. Whereas in normal B lymphocytes Lyn activation is dependent on B cell-receptor stimulation, in resting malignant cells, the constitutive activity of the kinase accounts for high basal protein tyrosine phosphorylation and low responsiveness to IgM ligation. Addition of the Lyn inhibitors PP2 and SU6656 to leukemic cell cultures restores cell apoptosis, and treatment of malignant cells with drugs that induce cell apoptosis decreases both activity and amount of the tyrosine kinase. These findings suggest a direct correlation between high basal Lyn activity and defects in the induction of apoptosis in leukemic cells. They also support a critical role for Lyn in B-CLL pathogenesis and identify this tyrosine kinase as a potential therapeutic target.

A systematic analysis reveals that out of 20 protein kinases examined, specific for either Ser/Thr or Tyr, the majority are extremely sensitive to staurosporine, with IC,,, values in the low nanomolar range. A few of them however, notably protein kinases CKI and CK2, mitogen-activated protein (MAP) kinase and protein-tyrosine kinase CSK, are relatively refractory to staurosporine inhibition, exhibiting IC,,, values in the micromolar range. With all protein kinases tested. namely PKA, CKI, CK2, MAP kinase (ERK-I), c-Fgr, Lyn, CSK and TPK-IIB/p38.'", staurosporine inhibition was competitive with respect to ATP, regardless of its inhibitory power. In contrast, either uncompetitive or noncompetitive kinetics of inhibition with respect to the phosphoacceptor substrate were exhibited by Ser/Thr and Tyr-specific protein kinases, respectively, consistent with a different mechanism of catalysis by these two sub-families of kinases. Computer modeling based on PKA crystal structure in conjunction with sequence analysis suggest that the low sensitivity to staurosporine of CK2 may be accounted for by the bulky nature of three residues, Va166, P h e l l 3 and Ile174 which are homologous to PKA Ala70, Met120 and Thr183, respectively. In contrast these PKA residues are either conserved or replaced by smaller ones in protein kinases highly sensitive to staurosporine inhibition. On the other hand, His160 which is homologous to PKA Glu170, appears to be responsible for the unique behaviour of CK2 with respect to a staurosporine derivative (CGP44171A) bearing a negatively charged benzoyl substituent: while CGP44171A is 10-100-fold less effective than staurosporine against PKA and most of the other protein kinases tested, it is actually more effective than staurosporine for CK2 inhibition, but it looses part of its efficacy if it is tested on a CK2 mutant (H160D) in which His160 has been replaced by Asp.It can be concluded from these data that the catalytic sites of protein kinases are divergent enough as to allow a competitive inhibitor like staurosporine to be fairly selective, a feature that can be enhanced by suitable modifications designed based on the structure of the catalytic site of the kinase.

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.

Highly purified rat liver mitochondria (RLM) when exposed to tert-butylhydroperoxide undergo matrix swelling, membrane potential collapse, and oxidation of glutathione and pyridine nucleotides, all events attributable to the induction of mitochondrial permeability transition. Instead, RLM, if treated with the same or higher amounts of H 2 O 2 or tyramine, are insensitive or only partially sensitive, respectively, to mitochondrial permeability transition. In addition, the block of respiration by antimycin A added to RLM respiring in state 4 conditions, or the addition of H 2 O 2 , results in O 2 generation, which is blocked by the catalase inhibitors aminotriazole or KCN. In this regard, H 2 O 2 decomposition yields molecular oxygen in a 2:1 stoichiometry, consistent with a catalatic mechanism with a rate constant of 0.0346 s ؊1 . The rate of H 2 O 2 consumption is not influenced by respiratory substrates, succinate or glutamate-malate, nor by N-ethylmaleimide, suggesting that cytochrome c oxidase and the glutathione-glutathione peroxidase system are not significantly involved in this process. Instead, H 2 O 2 consumption is considerably inhibited by KCN or aminotriazole, indicating activity by a hemoprotein. All these observations are compatible with the presence of endogenous heme-containing catalase with an activity of 825 ؎ 15 units, which contributes to mitochondrial protection against endogenous or exogenous H 2 O 2 . Mitochondrial catalase in liver most probably represents regulatory control of bioenergetic metabolism, but it may also be proposed for new therapeutic strategies against liver diseases. The constitutive presence of catalase inside mitochondria is demonstrated by several methodological approaches as follows: biochemical fractionating, proteinase K sensitivity, and immunogold electron microscopy on isolated RLM and whole rat liver tissue.Many human diseases, including cancer and other pathologies associated with aging, such as arteriosclerosis and cataracts, are related to mitochondrial dysfunctions provoked by reactive oxygen species (ROS) 2 (1). In this regard, the so-called free radical theory of aging has been proposed (2). ROS are highly reactive and may be extremely toxic in biological systems, as they attack a variety of molecules, including proteins, polyunsaturated lipids, and nucleic acid (3), causing the cell to die by apoptosis or necrosis. In physiological conditions, 1-2% of molecular oxygen consumption during mitochondrial respiration undergoes incomplete reduction by single electrons to form superoxide anion (O 2 . ) at the level of NADH-ubiquinone reductase (complex I) and ubiquinol-cytochrome c reductase (complex III). These two segments of the respiratory chain generate the superoxide radical by autoxidation of reduced flavin and by transferring an electron from reduced ubisemiquinone to molecular oxygen, respectively (4). Superoxide is rapidly converted to hydrogen peroxide by mitochondrial superoxide dismutase, which then produces the highly reactive hydroxyl radical (OH ⅐ ...

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