Tyrosine protein kinase activity of rat spleen and other tissues

Abstract

Using a synthetic peptide (Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg-Arg-Arg-Gly) as a substrate, various normal tissues from the rat were probed for tyrosine protein kinase activity. Spleen was shown to contain much higher tyrosine protein kinase activity than other rat tissues (lung, brain, testes, liver, kidney, heart, and thymus, in decreasing order of specific activity). Most of the tyrosine protein kinase activity of the various rat tissues (>80%) was present in the particulate fraction, and Nonidet P-40, a nonionic detergent, could activate the particulate form of the enzyme 2-20-fold in many of the tissues. Epidermal growth factor (1 microgram/ml), cyclic AMP, cyclic GMP, or Ca²⁺ did not increase spleen tyrosine protein kinase activity. Half-maximal enzyme activity was observed at 60-80 μ M MgATP and at 2.2 mM peptide, and both Mg²⁺ (10 mM) and Mn²⁺ (0.5-1.0 mM) were effective divalent metal ions for the expression of activity. When the particulate fraction of spleen was incubated with [γ⁻³²P]ATP followed by polyacrylamide gel electrophoresis in the presence of Na dodecyl SO₄, a number of alkali-stable bands were identified by autoradiography. Two major bands at M_r=53,000 and 56,000 were shown to contain phosphotyrosine. Two similar alkali-stable bands containing phosphotyrosine but with lower amounts of ³²P labeling were also observed in the particulate fractions of various other tissues (lung, brain, kidney, and testes). The particulate form of tyrosine protein kinase of rat spleen could be solubilized by using high concentrations of Nonidet P-40 (5%) at an alkaline pH (pH 9.0). Partial purification and subsequent filtration on Sephacryl S-200 yielded a peak of tyrosine protein kinase activity with an apparent molecular weight of 55,000. The two major phosphorylated bands of M_r=53,000 and 56,000 co-migrated with the peak of enzyme activity. The solubilized and partially purified enzyme preparation phosphorylated only tyrosine residues when either endogenous proteins or casein were used as substrates. These results suggest that relatively high activities of tyrosine protein kinase exist in a normal tissue (rat spleen). Major endogenous substrates of the enzyme(s) appear to be represented by two proteins of M_r=53,000 and 56,000; one or both of these substrates may be the tyrosine protein kinase itself.