Adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) which catalyzes the deribosylation of N'-(A2-isopentenyl)adenosine and adenosine to form the corresponding bases was partially purified from wheat gern. This enzyme (molecular weight 59,000 ± 3,000) deribosylates the ribonucleosides at an optimum pH of 4.7. Km values for the cytokinin nucleoside and esine are 238 and 1.43 micromolar, respectively, in 50 milHolar Tris-citrate buffer (pH 4.7) at 30 C. The presence of adenosine and other cytokinn nucleosides inlhbited the hydrolysis of N64A'.isopentenyl)adenosine but this reaction wa insensitive to guanosine, uridine, or 3'deoxy oine. It is hypothesized that an adequate level of "active cytokinin" in plant cells may be provided through the deribosylation of cytokinin riboside in concert with other cytokinin metabolic enzymes.Cytokinin bases and cytokinin ribonucleosides have been found in various plant cells, and these cytokinins are metabolized in the plant cell to form different metabolites (3,4,9,11,16,18). The relative amount of a specific metabolite formed may differ not only from one plant to another, but also in one particular plant or tissue under different physiological conditions. One of the major metabolites formed from a cytokinin ribonucleoside has been reported to be its corresponding base (9, 11, 13). In the de novo biosynthesis of cytokinins using a crude enzyme system prepared from plant cells, a cytokinin nucleotide, nucleoside, and base were formed from 5'-AMP and A2-isopentenylpyrophosphate (5). These observations indicate that in plant cells there are enzyme systems catalyzing the formation of cytokinin base from its nucleoside, which in turn can be formed from the corresponding nucleotide (7). Although hydrolytic conversion of Ado2 to Ade by adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) has been shown to occur in plant (10,14,17) and microbial (20) cells, the role of this enzyme in cytokinin metabolism has not been defined.We describe here the partial purification of adenosine nucleosidase from wheat germ, the properties of the enzyme, and the kinetics of the deribosylation of cytokinin ribonucleoside by this enzyme system. ' This work was supported by the National Science Foundation Re-search Grant PCM 79 03832. 2Abbreviations: Ado, adenosine; Ade, adenine; i6Ado, N6-(A2-isopentenyl)adenosine; i?Ade, N6-(A2-isopentenyl)adenine; t-io6Ado, trans-6-(4-hydroxy-3-methyl-2-butenylamino)-9-B-D-ribofuranosylpurine. Extraction and Fractionation of Enzyme. Wheat germ (135 g) frozen with liquid N2 was homogenized in a Waring Blendor in 10 mM Tris-HCI buffer (pH 7.0) (4 volumes/weight). The homogenate was filtered through double layers of cheesecloth. The filtrate was centrifuged for 10 min at 10,000g and the resulting supernatant was centrifuged again for 25 min at 20,000g. The supernatant is referred to as crude extract. The following steps were employed to further purify the extract:Step 1: Low pH Fractionation. The extract was brought to 30 C and its pH was adjust...

Two forms (F-I and F-II) of 5'-nucleotidases (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) which catalyze the dephosphorylatlon of N-(A2-lsopentenyl)adenosine S'-monophosphate and AMP to form the corresponding nucleosides were partially purified from the cytosol of wheat (Ticwum aesiwm) germ. Both the F-I (molecular weight, 57,000) and F-II (molecular weight, 110,000) 5'-nucleotidases dephosphorylate the ribonucleotides at an optimum pH of7. The Km values for the cytokinin nucleotide are 3.5 micromolar (F-I enzyme) and 12.8 micromolar (F-II enzyme) in 100 millmolar Tris-maleate buffer (pH 7) at 37 C. The F-I enzyme is less rapidly inactivated by heating than is the F-II enzyme. Both nucleotidases hydrolyze purine ribonucleoside S'-phosphates, AMP being the preferred substrate. N6-(A2-isopentenyl)Adenosine 5'-monophosphate is hydrolyzed at a rate 72 and 86% that of AMP by the F-I and F-II nucleotides, respectively. Phenylphosphate and 32-AMP are not substrates for the enzymes. It is proposed that dephosphorylation of cytokinin nucleotide by cytosol 5'-nucleotidases may play an important role in regulating levels of "active cytokinin" in plant cells.The occurrence of cytokinin nrbonucleotide and cytokinin ribonucleoside in plant cells is well documented (2,4,5,10,11).The cytokinin ribonucleotide can be formed from cytokinin base (3,11,18,20), the ribonucleoside (3,7,17,18) or turnover of cytokinin-containing tRNA (13,22). Alternatively, the ribonucleotide i6Ado-5'-P2 can be synthesized by a de novo pathway from the simple metabolites AMP and A2-isopentenylpyrophosphate (6, 24). In the de novo biosynthetic pathway using a crude enzyme preparation, cytokinin ribonucleoside was also formed. Thus, 5'-nucleotidases which are capable of dephosphorylating the ribonucleotide may be contained in the crude enzyme preparation. The 5'-nucleotidase has been isolated from animal cell membrane (8, 9, 14) and cytosol3 (19). Although the precise physiological function of this enzyme system in plant and animal cells is still unclear, the 5'-nucleotidase may play a role in the regulation of cytokinin metabolism.We describe here the partial purification of 5'-nucleotidase from wheat germ cytosol, the properties of the enzyme, and kinetics of 1 This work was supported by National Science Foundation Research Grant PCM 79 03832 (to C.-M. C).2Abbreviations: i6Ado-5'-P, N6-(A2-isopentenyl)adenosine 5'-monophosphate; Ade, adenine; Ado, adenosine; i6Ado, N6-(A2-isopentenyl)adenosine.3 The term "cytosol" is used only in distinction to "membrane bound." The extraction procedures do not rigorously preclude the possibility of contamination by nucleus enzymes. the dephosphorylation of cytokinin ribonucleotide by this enzyme system. MATERIALS AND METHODS CHEMICALS AND ENZYMESAdo, AMP, i6Ado, concanavalin A (grade IV), carbowax (mol wt, 6,000), 5'-nucleotidase (Crotalus adamteus venom), and wheat (Triticum aestivum) germ were from Sigma; i6Ado-5'-P was from P-L Biochemical Co., and [8-'4C]Ado (59 mCi/mmol) was from Amersham-Searle Corp....

Platelets from patients with Montreal platelet syndrome (MPS) consistently display a defect in the mechanisms that regulate platelet size during shape change and undergo spontaneous aggregation and stir- induced microaggregate formation. We now provide data that the surface glycoprotein composition of MPS platelets is indistinguishable from that of normal platelets. However, a defect in calcium-activated neutral proteinase (calpain) was detected in MPS platelets. The specific activity of calpain in the cytosolic fraction of platelets from four MPS patients was found to be only 30% of that in platelets from normal control donors (n = 18, P less than .001). Additionally, platelets from MPS patients (n = 3) contained only 50% (P less than .001) of the calpain I catalytic subunit antigen found in platelets from normal control donors (n = 9). Platelets from the asymptomatic father/grandfather of the MPS patients had normal amounts of both total calpain proteolytic activity and calpain I catalytic subunit antigen. This represents the first report of a defect in calpain in human cells. The abnormally low calpain activity in MPS platelets may account for the platelet defects characteristic of this disorder.

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An electroimmunoassay was applied to the quantitation of platelet- associated IgG (PAIgG). Protein solubilized by Triton X100 from washed platelets was electrophoresed at pH 5.0 in agarose gels containing carbamoylated rabbit anti-human IgG (pI approximately equal to 5.0). Because the rabbit antibody is immobilized under these conditions, while PAIgG is freely mobile, rocket precipitates were produced, the heights of which were directly proportional to the amount of antigen (PAIgG) present. By this method, PAIgG for normal individuals was found to be 4.3 +/- 1.7 fg/platelet (mean +/- 2 SD; n = 35). Increased PAIgG levels (direct assay) were found in 27 of 29 patients with a diagnosis of clinically active idiopathic thrombocytopenic purpura (ITP), ranging from 10.5 to 101.5 fg/platelet. Moderately elevated PAIgG was found in 8 of 10 patients in an early stage of recovery from ITP (range 7.5–9.5 fg/platelet) and in 3 of 6 patients with apparent nonimmune thrombocytopenia (range 14.5–24.0 fg/platelet). Electroimmunoassay for PAIgG can be performed on patients with platelet counts as low as 2000/microliters, yields results in less than 24 hr, is highly reproducible, and appears to provide a useful tool for the evaluation of patients with immunologically mediated thrombocytopenia.