Cellular compartmentalization of protein kinase activity during the cell cycle

The quantities and types of protein kinases found in the cytoplasmic and nuclear or chromosomal compartments of interphase and mitotic human culture cells were compared. Using histone as substrate, the total quantity of kinases recovered from cytoplasmic and chromosomal fractions of mitotic cells was several times greater than from cytoplasmic and nuclear fractions of interphase cells. In both mitotic and interphase cells, more activity was recovered from cytoplasmic fractions than from chromosomal or nuclear fractions, respectively. When activity against various substrates was examined, mitotic chromosomal extracts were found to display the greatest preference for the H1 fraction of histones. Neither cytoplasmic nor chromosomal fractions from mitotic cells exhibited enhanced activity in the presence of cAMP, whereas the activity of both cytoplasmic and nuclear fractions of interphase cells was enhanced. Protein kinases, previously identified by nondenaturing polyacrylamide gel electrophoresis as present in the cytoplasmic fraction of mitotic but not interphase cells, were also present in chromosomal fractions of mitotic cells; only one of these kinases may be present in nuclear extracts of interphase cells. In addition, the profiles of nuclear extracts of interphase cells differ from their cytoplasmic fractions. These results indicate that there are protein kinases which are restricted to the mitotic phase of the cell cycle and that they apparently partition between the cytoplasmic and chromosomal compartments of cells in mitosis.     

Nuclear protein kinase activities during the cell cycle of HeLa S3 cells

To ascertain the activity and substrate specificity of nuclear protein kinases during various stages of the cell cycle of HeLa S3 cells, a nuclear phospho-protein-enriched sample was extracted from synchronised cells and assayed in vitro in the presence of homologous substrates. The nuclear protein kinases increased in activity during S and G2 phase to a level that was twice that of kinases from early S phase cells. The activity was reduced during mitosis but increased again in G1 phase. When the phosphoproteins were separated into five fractions by cellulose-phosphate chromatography each fraction, though not homogenous, exhibited differences in activity. Variations in the activity of the protein kinase fractions were observed during the cell cycle, similar to those observed for the unfractionated kinases. Sodium dodecyl sulfate polyacrylamide gel electrophoretic analysis of the proteins phosphorylated by each of the five kinase fractions demonstrated a substrate specificity. The fractions also exhibited some cell cycle stage-specific preference for substrates; kinases from G1 cells phosphorylated mainly high molecular weight polypeptides, whereas lower molecular weight species were phosphorylated by kinases from the S, G2 and mitotic stages of the cell cycle. Inhibition of DNA and histone synthesis by cytosine arabinoside had no effect on the activity or substrate specificity of S phase kinases. Some kinase fractions phosphorylated histones as well as non-histone chromosomal proteins and this phosphorylation was also cell cycle stage dependent. The presence of histones in the in vitro assay influenced the ability of some fractions to phosphorylate particular non-histone polypeptides; non-histone proteins also appeared to affect the in vitro phosphorylation of histones.     

Phosphorylation of proteins in cytoplasmic and nuclear fractions of human cells treated with double-helical RNA and human recombinant interferon type I]

The dynamics of protein kinases activity in nuclear and cytoplasmic fractions of human fibroblasts treated by preparations of natural and synthetic dsRNA (ridostin, rifastin, larifan and poly(I).poly(C), DEAE-dextran and dsRNA complexes with DEAE-dextran), as well as by preparations of recombinant alpha-2 and beta-1 interferons was obtained. The early activation of enzymes in treated cells extracts and their presence in dsRNA-activated and nonactivated forms were found. In cytoplasmic cellular fractions treated by interferons the dsRNA dependent protein kinases (nonactivated forms- were prevalent.r In contrast, in dsRNA treated cells or dsRNA complexes with DEAE-dextran treated ones the dsRNA independent protein kinases (activated forms) were found, while dsRNA dependent forms induced by interferons were found at later periods. Nuclear protein kinases are mainly dsRNA independent making possible the supposition of their intracellular activation by incoming dsRNA or interferon-induced formation of ds-structures in cellular nuclei. In phosphorylated proteins spectre the 90, 69, 45-40 and 30-35 kDa polypeptides were found. At early intervals in nuclear fractions was found a nuclease resistant and partially EDTA resistant high molecular phosphorylated complex (120 kDa). The complex is, probably, capable of dissociation to low mol mass components. DEAE-dextran induces strong activation of protein kinases in cytoplasm and nuclei and increases the content of activated forms of enzyme in larifan treated cells.     

Nuclear accumulation of multiple protein kinases during prolactin-induced proliferation of Nb2 rat lymphoma cells

Intracellular kinases play important roles in signal transduction and are involved in the surface receptor-mediated regulation of cellular functions, including mitogenesis. In the present study, we examined the possible involvement of various protein kinases in the passage of a mitogenic signal from the cell surface to the nucleus of Nb₂ cells, a rat nodal lymphoma cell line in which prolactin is a mitogen. Following a prolactin challenge, various kinase activities were monitored at short intervals in different cellular fractions over a 60 min period. Protein kinase C (PKC) activity in the cytosolic fraction rapidly declined to 50% of its original activity within the first 30 min, while PKC activity in the nuclear fractions increased sharply, reaching its highest level by 30 min following a prolactin challenge. There were also increases in both casein kinase and protein tyrosine kinase (PTK) activities in the nuclear fractions during the first 30 min following a prolactin challenge that paralleled PKC activity. The activities of all three kinases declined thereafter, reaching levels close to their respective basal values by 60 min following initiation of prolactin treatment. These observations suggest the possibility that multiple protein kinases may be involved in mitogenic signal transduction for prolactin in Nb₂ cells. © 1996 Wiley-Liss, Inc.     

Cross-linking of SIGNR1 activates JNK and induces TNF-α production in RAW264.7 cells that express SIGNR1

In this study, we evaluated the signaling ability of SIGNR1 in murine macrophage-like RAW264.7 cells that stably expressed FLAG-tagged SIGNR1 (SIGNR1-FLAG). Cross-linking of SIGNR1-FLAG expressed on the cells by an anti-FLAG antibody induced JNK phosphorylation without induction of phosphorylation of ERK1/2 and p38 MAP kinase, and led to phosphorylations of Src family kinases (SFKs) and Akt. The SIGNR1-FLAG molecules in the cells were found in lipid raft-enriched membrane fractions, and the tyrosine kinases Lyn, Hck, and Fgr co-precipitated with SIGNR1-FLAG in the lipid raft fractions. The antibody-induced JNK phosphorylation was inhibited by inhibitors of SFKs and tyrosine kinases. Furthermore, cross-linking of SIGNR1 led to production of TNF-α, and the JNK inhibitor inhibited the antibody-induced TNF-α production. These results show that cross-linking of SIGNR1 triggers phosphorylation of SFKs, which leads to activation of the JNK pathway and induction of TNF-α production in macrophage-like RAW264.7 cells.     

Association of phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase activities with the cytoskeleton in human platelets

The inositol lipid kinases were investigated in the cytoskeletons of human platelets. In the absence of added lipids the kinases were only barely detectable in the Triton-soluble fractions and undetectable in cytoskeletons of resting cells. However at least 30% of the total phosphatidylinositol kinase was present in the cytoskeleton as revealed by saturation of the enzyme. Phosphatidylinositol 4-phosphate kinase was also found in significant amounts in the cytoskeletons. On the other hand, both enzymes being only recovered in the particulate fraction of the cells, we suggest that inositol lipid kinases may be present near the anchoring points of the cytoskeletons at the membranes.     

Detection of protein kinase substrates in tissue extracts after separation by isoelectric focusing

Here we report a simple and useful method to detect endogenous substrates of protein kinases. When crude tissue extracts were resolved by liquid-phase isoelectric focusing (MicroRotofor) and the separated protein fractions were phosphorylated by protein kinases such as Ca²⁺/calmodulin-dependent protein kinase I or cAMP-dependent protein kinase, various proteins in the different fractions were efficiently phosphorylated. Since a higher number of substrates could significantly be detected using the resolved fractions by MicroRotofor as compared to direct analysis of the original tissue extracts, our present method will be applicable to the screening of endogenous substrates for various protein kinases.     

Parietal cell protein kinases. Selective activation of type I cAMP-dependent protein kinase by histamine

cAMP-dependent protein kinases have been characterized in parietal cells isolated from rabbit gastric mucosa. Both Type I and Type II cAMP-dependent protein kinase isozymes are present in these cells. Type II isozymes were detected in 900, 14,000, and 100,000 X g particulate fractions as well as 100,000 X g cytosolic fractions; Type I isozymes were found predominately in the cytosolic fraction. When parietal cells were stimulated with histamine, an agent that elevates intracellular cAMP content and initiates parietal cell HCl secretion, cAMP-dependent protein kinase activity was increased in homogenates of these cells as measured by an increase in the cAMP-dependent protein kinase activity ratio. Histamine activation of cAMP-dependent protein kinase was correlated with parietal cell acid secretory responses which were measured indirectly as increased cellular uptake of the weak base, [14C]aminopyrine. These results suggest that cAMP-dependent protein kinase(s) is involved in the control of parietal cell HCl secretion. The parietal cell response to histamine may be compartmentalized because histamine appears to activate only a cytosolic Type I cAMP-dependent protein kinase isozyme, as determined by three different techniques including 1) ion exchange chromatography; 2) Sephadex G-25 to remove cAMP and allow rapid reassociation of the Type II but not the Type I isozyme; and 3) 8-azido-[32P]cAMP photoaffinity labeling. Forskolin, an agent that directly stimulates adenylate cyclases, was found to activate both the Type I and Type II isozymes. Several cAMP-dependent protein kinases were also detected in parietal cell homogenates, including a Ca2+-phospholipid-sensitive or C kinase and two casein kinases which were tentatively identified as casein kinase I and II. At least two additional protein kinases with a preference for serine or lysine-rich histones, respectively, were also detected. The function of these enzymes in parietal cells remains to be shown.     

Cyclic AMP-dependent protein kinases from Balb 3T3 cells and other 3T3 derived lines

Cyclic AMP-dependent protein kinase and 3H-cAMP-binding activities were determined in normal Balb 3T3 cells and compared with the same preparations from SV40, chemical, and spontaneous transformants of 3T3 cells. The cytosolic protein kinase activities and protein kinase activity ratios were similar in all cell lines, although when the normal 3T3 cytosol was prepared by homogenization it contained less 3H-cAMP binding activity than the transformed 3T3 cytosols. The Triton X-100 treated particulate fractions from the normal and transformed 3T3 cells contained similar protein kinase and binding activities. The isozymic profile of cAMP-dependent protein kinases was examined by DEAE-chromatography. The 3T3 cells contained only type II isozyme in either cytosolic or membrane fractions. All transformants of the 3T3 cells contained both type I and type II isozymes. Other cell cultures, including chicken embryo fibroblasts, rat kidney cells, and human or calf endothelial cells contained type I and type II isozymes. Binding of the photoaffinity analogue of cAMP, 8-N3 cAMP, to the regulatory subunits of protein kinases in sonicates obtained from Balb 3T3 and SV 3T3 cells followed by separation on SDS polyacrylamide electrophoresis showed that the amount of RII subunit was approximately equal in the two cell lines. RI in Balb 3T3 cells was detectable but in a much lower quantity than in SV 3T3 cells. The cyclic AMP dependent-protein kinases from Balb 3T3 cells appears to be different from SV 3T3 cells by three criteria: 3H-cAMP binding in homogenates, DEAE chromatographic separation of isozymes, and 8-N3 cAMP binding.     

Integration of conventional quantitative and phospho‐proteomics reveals new elements in activated Jurkat T‐cell receptor pathway maintenance

Recent years have seen a constant development of tools for the global assessment of phosphoproteins. Here, we outline a concept for integrating approaches for quantitative proteomics and phosphoproteomics. The strategy was applied to the analysis of changes in signalling and protein synthesis occurring after activation of the T‐cell receptor (TCR) pathway in a T‐cell line (Jurkat cells). For this purpose, peptides were obtained from four biological replicates of activated and control Jurkat T‐cells and phosphopeptides enriched via a TiO₂‐based chromatographic step. Both phosphopeptide‐enriched and flow‐through fractions were analyzed by LC–MS. We observed 1314 phosphopeptides in the enriched fraction whereas 19 were detected in the flow‐through, enabling the quantification of 414 and eight phosphoproteins in the respective fractions. Pathway analysis revealed the differential regulation of many metabolic pathways. Among the quantified proteins, 11 kinases with known TCR‐related function were detected. A kinase‐substrate database search for the phosphosites identified also confirmed the activity of a further ten kinases. In total, these two approaches provided evidence of 19 unique TCR‐related kinases. The combination of phosphoproteomics and conventional quantitative shotgun analysis leads to a more comprehensive assessment of the signalling networks needed for the maintenance of the activated status of Jurkat T‐cells.     

Characterization of distinct tyrosine-specific protein kinases in B and T lymphocytes

Lymphocyte membrane fractions from both normal and neoplastic sources exhibit tyrosine-specific protein kinase activity. The molecular weights of the endogenous substrates phosphorylated on tyrosine residues differ in B and T cells. To further characterize membrane tyrosine phosphorylation in the two major classes of lymphocytes, the tryptic phosphopeptides of their endogenous substrates were compared and the sensitivity of the kinases to inhibition by N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) was determined. The two major B cell substrates (61,000 and 55,000 daltons, p61 and p55) were gel purified after phosphorylation and exhaustively digested with trypsin. Separation by reverse phase high pressure liquid chromatography demonstrated that these two substrates had two identical phosphotyrosine containing tryptic phosphopeptides. p61 had an additional phosphotyrosine site. Parallel analysis of the two T cell substrates (64,000 and 58,000 daltons, p64 and p58) showed that they also contained two phosphotyrosine sites that were identical. However, the tryptic phosphopeptides from the B and T cell substrate pairs were clearly distinct suggesting that they arise from different gene products. When B and T cell membrane fractions were preincubated with TLCK (21 degrees C, 30 min) a dose-dependent decrease in p64 and p58 phosphorylation resulted. p61 and p55 phosphorylation was not affected at concentrations up to 10 mM TLCK. Tyrosine-specific kinase activity was also assessed by measuring phosphorylation of a tyrosine containing synthetic peptide. The kinase activity of T cell plasma membrane fractions was inhibited by TLCK; the B cell activity was unaffected. The results suggest that membrane fractions from normal and some neoplastic B and T cells have at least two different tyrosine-specific kinases.     

In situ phosphorylation of tyrosine hydroxylase in chromaffin cells: Localization to soluble compartments

The present studies investigated the subcellular distribution of acetylcholine's effects upon the phosphorylation of tyrosine hydroxylase in isolated purified bovine adrenal chromaffin cells. After labeling the intact chromaffin cells with ³²P_i, over 90% of the [³²P]tyrosine hydroxylase was found in soluble fractions. Stimulation of the cells with acetylcholine, the natural secretagogue of chromaffin cells, increased the phosphorylation of tyrosine hydroxylase and over 90% of the increase was associated with soluble tyrosine hydroxylase. Homogenates and subcellular fractions from chromaffin cells were also prepared and phosphorylated in vitro in an attempt to optimize detection of tyrosine hydroxylase phosphorylation. In chromaffin cell homogenates, both 8-bromo-cyclic AMP and calcium increased ³²P incorporation into tyrosine hydroxylase, and again over 90% of the increase was observed in soluble fractions. In the particulate fraction, phosphorylation of a band which comigrated with tyrosine hydroxylase in electrophoresis was occasionally detected but only with very long autoradiographic exposures.Tyrosine hydroxylase enzymatic activity in the isolated purified chromaffin cells was also found to be associated predominantly (approx 90%) with soluble fractions. In contrast, a large portion (40–50%) of the tyrosine hydroxylase activity from crude bovine adrenal medullae was associated with the particulate fraction.The data indicate that although tyrosine hydroxylase (and possibly kinases) can associate with particulate fractions when isolated from crude bovine adrenal medullae, the enzyme is predominantly soluble when isolated from the isolated cells. Further, the effects of acetylcholine on the isolated chromaffin cells are predominantly associated with this soluble tyrosine hydroxylase and its attendant kinases.     

Identification of multiple protein kinases in normal human lymphocytes.

The protein kinase activity of a 10,000 g supernatant of purified human lymphocytes can be resolved by DEAE-cellulose chromatography into six protein kinase fractions: three of them phosphorylate casein preferentially, and three histones. The same procedure with the corresponding nuclear fraction yields only two casein kinases. All these fractions, except one casein kinase of the cytosol, have been studied with respect to protein and nucleotide specificity, effect of salts and of cyclic nucleotides, sedimentation, etc. The results obtained indicate that the enzyme fractions of the cytosol have distinct characteristics, suggesting that they are different protein kinases, and that the nuclear kinases are similar to the two main casein kinases of the cytosol.     

Studies on protein kinases in two human rectocolic cell lines by polyacrylamide gel electrophoresis.Effect of the vasoactive intestinal peptide

Electrophoresis and subsequent assay of the enzyme directly onto the gel has allowed a rapid and quantitative characterization of the cyclic AMP-dependent and -independent histone kinases, protamine, phosvitin and casein kinases in HT 29 and HRT 18 cells. The technique has been applied to soluble extracts from cytoplasmic and nuclear fraction prepared in the presence and absence of neutral detergent. A more precise identification of these enzymes has been possible by analysing enzyme fractions obtained after ion-exchange chromatography of the above extracts. The protein kinase equipment of both cell lines was found to be identical (11 major components) but with different relative proportions of several enzymes. In cytoplasmic extracts: VIP activates only the type I, cytosolic, (band 4) and the type II, membrane-bound, (bands 6 and 8) cyclic AMP-dependent histone kinases. These enzymes account, respectively, for 34 and 55% of the total histone kinases in HT 29 and HRT 18 cells. The cyclic AMP-independent histone kinases (band 1,2,5 and 7) also phosphorylate protamine; band 5 was found 3o be much higher (4-fold) in HT 29 cells. In addition, two casein/phosvitin kinases have been identified in both cell lines with phosphorylating activity similar to the total histone kinases. In the nuclear extract two cyclic AMP-independent histone kinases have been found with electrophoretic mobility differing from the cytoplasmic enzymes. Also, two phosvitin/casein kinases specifically nuclear, due to their chromatographical and electrophoretical behaviour, have been characterized.     

Localization of catalytic and regulatory subunits of cyclic AMP-dependent protein kinases in mitochondria from various rat tissues.

Observation and quantification of the catalytic subunit C of cyclic AMP-dependent protein kinases by immuno-gold electron microscopy suggested a high concentration of cyclic AMP-dependent protein kinases in mitochondria from liver, kidney, heart and skeletal muscle, pancreas, parotid gland and brain cells. The position of gold particles pointed to a localization in the inner membrane/matrix space. A similar distribution was obtained by immunolocalization of the cyclic AMP-dependent protein kinase regulatory subunits RI and RII in liver, pancreas and heart cells. The results indicated the presence of both the type I and the type II cyclic AMP-dependent protein kinases in mitochondria of hepatocytes, and the preferential occurrence of the type I protein kinase in mitochondria from exocrine pancreas and heart muscle. The immunocytochemical results were confirmed by immunochemical determination of cyclic AMP-dependent protein kinase subunits in fractionated tissues. Determinations by e.l.i.s.a. of the C-subunit in parotid gland cell fractions indicated about a 4-fold higher concentration of C-subunit in the mitochondria than in a crude 1200 g supernatant. Immunoblot analysis of subfractions from liver mitochondria supported the localization in situ of cyclic AMP-dependent protein kinases in the inner membrane/matrix space and suggested that the type I enzyme is anchored by its regulatory subunit to the inner membrane. In accordance with the immunoblot data, the specific activity of cyclic AMP-dependent protein kinase measured in the matrix fraction was about twice that measured in whole mitochondria. These findings indicate the importance of cyclic AMP-dependent protein kinases in the regulation of mitochondrial functions.     

Regulation of NAD-dependent glutamate dehydrogenase by protein kinases in Saccharomyces cerevisiae

The active NAD-dependent glutamate dehydrogenase of wild type yeast cells fractionated by DEAE-Sephacel chromatography was inactivated in vitro by the addition of either the cAMP-dependent or cAMP-independent protein kinases obtained from wild type cells. cAMP-dependent inhibition of glutamate dehydrogenase activity was not observed in the crude extract of bcy1 mutant cells which were deficient in the regulatory subunit of cAMP-dependent protein kinase. The cAMP-dependent protein kinase of CYR3 mutant cells, which has a high K alpha value for cAMP in the phosphorylation reaction, required a high cAMP concentration for the inactivation of NAD-dependent glutamate dehydrogenase. An increased inactivation of partially purified active NAD-dependent glutamate dehydrogenase (Mr = 450,000) was observed to correlate with increased phosphorylation of a protein subunit (Mr = 100,000) of glutamate dehydrogenase. The phosphorylated protein was labeled by an NADH analog, 5'-p-fluorosulfonyl[14C]benzoyladenosine. Activation and dephosphorylation of inactive NAD-dependent glutamate dehydrogenase fractions were observed in vitro by treatment with bovine alkaline phosphatase or crude yeast cell extracts. These results suggested that the conversion of the active form of NAD-dependent glutamate dehydrogenase to an inactive form is regulated by phosphorylation through cAMP-dependent and cAMP-independent protein kinases.     

细胞核CaMK和Calcineurin 对大鼠心肌肥厚发生的作用

目的:研究大鼠心肌肥厚时,钙依赖的蛋白激酶和蛋白磷酸酶在心肌细胞膜、细胞浆和细胞核的分布规律,以探讨核钙信号与核反应在心肌肥厚发生过程中的病理生理意义.方法:制备腹主动脉缩窄大鼠心肌肥厚模型,同位素32P掺入法分别测定心肌细胞核、细胞浆和细胞膜的蛋白激酶活性及用无机磷生成显色法测定其蛋白磷酸酶活性.结果:腹主动脉缩窄术后4周大鼠心肌显著肥厚,伴有明显的血液动力学异常.与正常对照组相比较,腹主动脉缩窄心肌肥厚组心肌细胞核钙调素蛋白激酶(CaMK)活性增加101.1%(P＜0.01),其膜的酶活性升高40.2%(P＜0.01),而胞浆的酶活性不变(P＞0.05);心肌细胞核钙调神经磷酸酶(Calcineurin)活性增加43.6%(P＜0.05),膜和胞浆中其活性增加无显著性(P＞0.05).正常组和腹主动脉缩窄心肌肥厚组心肌细胞CaMK和Calcineurin活性分布为核＞膜＞胞浆(P＜0.01).结论:腹主动脉缩窄心肌肥厚时核内钙依赖的CaMK和Calcineurin活性增加,提示压力超负荷时细胞核内钙调节的蛋白磷酸化和去磷酸化水平增高,可能在介导心肌肥厚的发生中起重要作用.     

Nuclear activation and translocation of mitogen-activated protein kinases modulated by ethanol in embryonic liver cells

Activation and nuclear translocation of mitogen activated protein (MAP) kinases in ethanol-treated embryonic liver cells (BNLCL2) was investigated. The relative amount of MAPK proteins, MAP kinase activity and MAPK/LDH (lactate dehydrogenase) ratios were determined in nuclear and cytosolic fractions before and after serum stimulation. In ethanol-treated cells, serum-stimulated MAPK activation was potentiated in both cytosolic and nuclear fractions. Levels of both the p42 and p44 MAPK proteins increased in nuclear fractions from cells treated with ethanol alone for 24 h. Serum-stimulated nuclear translocation of both p42 and p44 MAPK was potentiated in ethanol-treated cells. Nuclear fractions from ethanol-treated cells had a modest increase in MAP kinase activity concurrent with the increased MAPK protein levels. The ratio of MAPK/LDH increased in nuclear fractions with increasing concentrations of ethanol and after serum stimulation. This further confirmed the nuclear translocation of MAPK and also demonstrated that it is not a non-specific effect of ethanol. These results demonstrate, for the first time, that in BNLCL2 liver cells ethanol treatment has dual effects. First, ethanol triggered nuclear translocation of MAPK without causing its activation. Second, it potentiated serum-stimulated activation and translocation of MAPK in the nucleus. These findings provide a novel mechanism through which ethanol may affect cellular and nuclear processes in liver cells.     

Several properties of 3':5'-AMP-dependent skeletal muscle protein kinases in normal rats and following physical exertion to fatigue]

Three fractions of rat adenosine-3',5'-monophosphate-dependent protein kinase were isolated, partially purified in buffer concentration gradient at normal state and after long-term physical loading and studied. It is found that first two fractions of protein kinases at normal state and after intensive muscular work have similar activities with and without cAMP, apparent Km values for ATP and total histone and half-maximal stimulation by cyclic AMP, but they differed from the third fraction. There are differences in some kinetic parameters and in the cyclic AMP stimulated activities between protein kinases after physical loading. The data obtained suggest the existence of at least two kinases in rat skeletal muscle. The isoenzymes differ in their activities during fatigue.