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Revista Brasileira de Fisiologia Vegetal

Print version ISSN 0103-3131

Rev. Bras. Fisiol. Veg. vol.13 no.3 Lavras  2001 




Laboratório de Fisiologia Vegetal, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, P.O. Box 6039, 60.455-900, Fortaleza, Ceará, Brazil



ABSTRACT - Partial purification and characterization of ribonucleases (RNase; EC present in roots, stem and leaves of 5 day-old Pitiúba cowpea [Vigna unguiculata (L.) Walp.] seedlings are described. Crude extracts from the different tissues were precipitated with ammonium sulfate followed by ionic exchange chromatography (CM-Cellulose) resulting in purification factors of 48-fold  for roots, 21 for stem and 42 for leaves. No deoxyribonuclease activity was practically observed. The molecular masses of the RNases did not significantly differ, averaging 16.3 kDa. Leaf RNase was stable up to 50ºC while the others were inactivated at this temperature. The maximal inactivation for both stem and roots RNases was reached at 70ºC while for leaf it occurred at 80ºC. The addition of KCl to the assay medium caused a shift of optimal pH from 6.0 toward the range of 5.2 - 5.6 for the enzymes extracted from the different tissues.  RNase activities were strongly inhibited by Hg2+, Zn2+ and Cu2+, partially inhibited by Co2+ and Fe2+ and were not affected by EDTA, Ca2+ or Mg2+. In contrast  to  the  leaf  RNase,  roots  and  stem  enzymes  were  inactivated  by  urea  and 2-mercaptoethanol (2-ME). Although there is a great similarity among the enzymes studied, leaf RNase appears to be more stable to heat and to chemical denaturation than root and stem RNases. The results also suggest that the enzymes extracted from different tissues of Pitiúba cowpea seedlings are ribonucleases and not nucleases. 
ADDITIONAL INDEX TERMS - enzyme effectors, molecular mass, nuclease, optimum pH,  seedlings, thermostability, Vigna unguiculata.


RESUMO - Este trabalho descreve a purificação parcial e caracterização das ribonucleases (RNase; EC presentes em raízes, caule e folhas de plântulas de feijão-de-corda Pitiuba [Vigna unguiculata (L.) Walp.].  Precipitação   dos  extratos brutos  dos  diferentes tecidos com sulfato de amônio, seguida de cromatografia  de  troca  iônica  (CM-Celulose),  resultaram  em  purificação  de  cerca de 48 vezes para a enzima de raízes, 21 para a de caule e 42 para a de folha. A atividade desoxirribonucleásica nas frações semi-purificadas foi praticamente nula. As massas moleculares das RNases não diferiram significativamente, sendo a média de suas massas 16,3 kDa. A RNase de folha mostrou-se estável até 50ºC enquanto que as outras foram inativadas nesta temperatura. A inativação máxima das RNases de raízes e de caule ocorreu a 70ºC, mas a da RNase foliar ocorreu a 80ºC. A adição de KCl ao meio de ensaio causou uma mudança no pH ótimo de 6,0 para a faixa de 5,2-5,6 para as enzimas extraídas dos diferentes tecidos. As atividades RNásicas foram fortemente inibidas por Hg2+, Zn2+ e Cu2+, inibidas parcialmente por Co2+ e Fe2+ e não foram afetadas por EDTA, Ca2+ ou Mg2+. Diferentemente do observado para a RNase foliar, as enzimas de raízes e de caule foram inativadas por uréia e 2-mercaptoetanol (2-ME). Embora exista grande semelhança entre as enzimas estudadas, a RNase foliar parece ser mais estável ao calor e à desnaturação química do que as RNases de raiz e de caule. Os resultados sugerem que as enzimas extraídas dos diferentes tecidos de plântulas de feijão-de-corda Pitiúba são ribonucleases e não nucleases.
TERMOS ADICIONAIS PARA INDEXAÇÃO: efetores da enzima, massa molecular, nuclease,  pH ótimo,  plântula, termoestabilidade, Vigna unguiculata.




Ribonucleases (RNase; EC have a key role on plant development due to their ability to modify RNA levels, and thereby influence protein synthesis (Tvorus, 1976). Hydrolysis and mobilization of RNA in seed tissues are important steps during germination and seedling establishment (Bewley & Black, 1994). Ribonucleases have previously been isolated from several organisms such as, fungi (Wang & Ng, 1999), higher plants (Gomes-Filho & Sodek, 1988; Nakagawa et al., 1999; Tanaka et al., 2000) and mammals (Pileur et al., 2000).  There are four types of enzymes that hydrolyze RNA in plants: RNase I, RNase II, nuclease I and exonuclease I (Wilson, 1975, 1982). While RNases uses only RNA as substrate and are insensitive to EDTA, nucleases hydrolyze both RNA and DNA and are highly affected by EDTA (Wilson, 1975, 1982; Green, 1994).

Cellular RNA level is a function of the balance between synthesis and degradation (Farkas, 1982) and plants are known to alter the levels of RNase activities in response to endogenous or exogenous stimuli (Farkas, 1982; Wilson, 1982; Gomes-Filho & Sodek, 1988; Dodds et al., 1996). However, the mechanisms involved in these processes are poorly understood (Gomes-Filho et al., 1999), and in order to understand these mechanisms it is fundamental to isolate and characterize the different types of RNases. Ribonuclease characteristics have been studied in the different Vigna unguiculata tissues during germination and seedling development (Gomes-Filho & Enéas-Filho, 1991), but only cotyledon RNase has been purified and characterized (Gomes-Filho et al., 1999). Furthermore, the name RNase has been used without knowing whether or not the activity measured is a mixture of nucleases and RNases because only RNA is generally tested as substrate.

Therefore, the objectives of the present paper were to determine possible differences among the RNases from different organs of cowpea seedlings and to clarify if they are true RNases or nucleases. RNases from roots, stem and leaves were partially purified and some of their kinetic and physical chemical properties studied. The enzymatic characteristics studied were: activity against DNA, molecular masses, heat stability, pH curves, and action of some effectors on the enzyme activities.



Plant material

Pitiúba cowpea [Vigna unguiculata (L.) Walp.] seeds were obtained from plants grown under irrigation at the Centro de Ciências Agrárias, Universidade Federal do Ceará, Fortaleza, Ceará, Brazil. Seeds were kept at 10ºC in glass flasks containing silica gel. Seeds, previously surface-sterilized with sodium hypochloride, were allowed to germinate on filter paper (Prisco & Vieira, 1976) at 25 ± 2ºC  in darkness.

Ribonuclease extraction and purification

Roots, stems and leaves from Pitiúba cowpea seedlings at 5 days after seeding were used to obtain RNases crude extracts. The different tissues were ground in  0.1 M potassium phosphate buffer, pH 5.7 (1:3; w/v), at 4ºC (Gomes-Filho & Enéas-Filho, 1991). The extracts were left without stirring for 30 min, passed through a nylon filter and centrifuged at 7000 x g for 20 min at 4ºC. The supernatant (crude extract) was precipitated with ammonium sulfate (40-80% saturation) and the pellet obtained after centrifugation was resuspended in 0.1 M sodium acetate buffer, pH 4.5, dialyzed against the same buffer overnight, and finally dialysed against cold distilled water for 24 h. This fraction (F40-80) was centrifuged and the supernatant was lyophilized and stored at 4ºC.

The lyophilized F40-80 was resuspended in cold 0.1 M sodium acetate buffer, pH 4.5, and applied to a carboxymethyl-cellulose (CM-Cellulose) column (18 x 175 mm) equilibrated with the same buffer at 10ºC. The flow rate was adjusted to 32 mL.h-1, and  after  washing  the  column  with  the  initial  buffer,  proteins  were  eluted  with a 0-1 M NaCl gradient (100 mL) and 3.5 mL fractions were collected. The fractions containing ribonuclease activity (FCM fraction) were pooled, dialyzed against cold distilled water and lyophilized for enzyme characterization.  

Protein was determined according to Bradford (1976), using bovine serum albumin  (Sigma Chemical Co.) as  standard  or  expressed  as absorbance at 280 nm (A280), for the chromatographic fractions.

Ribonuclease and deoxyribonuclease activity

RNase activity was assayed according to Tuve & Anfinsen (1960) with minor modifications (Gomes-Filho & Enéas-Filho, 1991), using yeast RNA type IV (Sigma Chemical Co.) as substrate. Deoxyribonuclease activity was determined according to Wilson (1968), using linear DNA from Salmon sperm (Sigma Chemical Co.) as substrate.  In both assays, the substrates were dissolved in 0.1 M sodium acetate buffer, pH 5.8 and the assay temperature was 40ºC. The reaction was terminated by addition of 0.75% uranyl acetate in HClO4 at 25%, and acid-soluble nucleotide production was determined spectrophotometrically at 260 nm. Enzyme activity was expressed as difference in absorbance (DA260).

Molecular mass

Molecular    masses    were    obtained    with    a    Sephadex   G-100    column  (25 x 320 mm) using 0.1 M sodium acetate buffer, pH 5.8. The column flux was adjusted to 32 mL.h-1 and fractions of 4.3 mL were collected. Protein and RNase activity were determined as described above. The molecular mass was calculated according to Determan (1969). The chromatographies were carried out in triplicate.

Ribonuclease properties

Thermostability was obtained with 0.2 mL FCM fractions in 0.1 M sodium acetate buffer, pH 5.8, preincubating at temperatures ranging from 30 to 100ºC, for 10 min, followed by cooling on ice bath and determining the residual enzyme activity. Three independent experiments were carried out, each one in triplicate.

Optimum pH of  FCM  was  assayed  in  sodium  acetate buffer (I = 0.05, pH 4.0 to 6.0) and in sodium phosphate buffer (I = 0.05, pH 6.4 to 7.6). Enzyme activity as a function of pH was determined both in the presence and absence of 0.16 M KCl. Three independent experiments were carried out, each one in triplicate.

The effect of divalent cations on the enzyme activity was investigated by adding chloride salts to the reaction mixture at 3.0 mM final concentration. The effects of urea and 2-mercaptoethanol (2-ME) were studied by preincubating FCM  fraction  with 6.0 M urea and 140 mM 2-ME for 1 h at 25ºC, and the enzyme activity assayed at a final concentration of 1.0 M urea and 140 mM of 2-ME. Three independent experiments were carried out, each one in duplicate.



Purification of ribonucleases

Ribonucleases from roots, stem and leaves of Pitiúba cowpea seedlings were purified 48-, 21- and 42-fold, respectively (Table 1). The CM-Cellulose chromatography (Figure 1A) has shown that the RNase from leaves was retained and eluted in a single peak with a NaCl gradient. Similar results were observed for roots and stems (data not shown). No deoxyribonuclease activities were practically detected in FCM fraction from roots, stems and leaves (data not shown). Therefore, these data suggest that the enzymes studied are RNases and not nucleases, as defined by Wilson (1975, 1982). Similar results were obtained with cotyledonary RNase from the same cultivar (Gomes-Filho & Sodek, 1988).




Ribonuclease properties

RNase  from  leaves,  roots  and  stems,  eluted  from  Sephadex  G-100  column, have  shown  a  single  peak  of  activity  with  an  average  molecular  mass  of  about 16.3 kDa (Figure 1B). Cotyledonary RNase extracted from the same cultivar has shown a  molecular mass of 16.0 kDa, determined by SDS-PAGE (Gomes-Filho et al., 1999) and RNases with molecular mass of 17.0 kDa were found in maize (Fennoy et al., 1997) and white lupin (Bantignies et al., 2000) roots. RNases with smaller molecular masses (8-14 kDa) have also been found in wheat seedlings (Zhao et al., 2000).

Thermostability measurements (Figure 2) showed that the ribonuclease from leaves was stable up to 50ºC, while stem and roots enzymes were inactivated at this temperature. The maximal inactivation for both stems and roots RNases was reached at 70ºC, while leaves RNase maximal inactivation occurred at 80ºC, suggesting that the latter enzyme was more thermostable. All RNases have shown a tendency to be more stable at temperatures above their maximal inactivation. Similar results were observed for wheat RNase (Hanson & Fairley, 1969) and for partially purified cowpea cotyledonary RNase (Gomes-Filho et al., 1994). However, when this cowpea RNase was purified this was not observed (Gomes-Filho et al., 1999), suggesting the presence of more thermostable contaminants.



The optima pH values for RNases  extracted  from  roots  (Figure 3A), stem (Figure 3B) and leaves (Figure 3C) were close to 6.0 when KCl was not present. The addition of KCl caused a shift of optimal pH toward the range of 5.2-5.6. The decrease in optima pH due to the presence of KCl suggest that they are similar to the RNase I extracted from maize scutellum (Wilson, 1963).



The effects of divalent cations and other effectors on RNases extracted from roots, stem and leaves are shown on  Table 2.  Ca2+ and Mg2+ caused a slight inhibition (lower than 17%), while Hg2+, Zn2+ and Cu2+ strongly inhibited the RNase activity (greater than 88%).  This strong inhibition due to heavy metals (Hg2+, Zn2+ and Cu2+) suggest that one or more free sulphydryl groups are present within the active site region of the enzyme (Biswas, 1987, Nelson & Cox, 2000). Fe2+ inhibited the different RNase activities to a varying extent:  41% in roots, 75% in stems and 64% in leaves. The effects of ions on the RNase activity were similar to those observed for the enzymes from rice seeds (Yokoyama et al., 1982) and sunflower roots (El-Shouny & Alexandrescu, 1986). EDTA caused little change in the RNase activities as also observed for cotyledonary RNases from cowpea (Gomes-Filho et al., 1994). Root and stem  RNases  were  inactivated by urea and 2-ME, while the leaf enzyme was insensitive to 2-ME and slightly stimulated by urea. The changes in root and stem RNases induced by urea and 2-ME are the result of changes in the tertiary structure of the enzyme, i.e., protein denaturation due to urea, and reduction of disulfide bridges due to 2-ME. These results and the higher thermostability found for the RNase extracted from leaves suggest that this protein is more resistant to denaturation and less dependent on disulfide bonds for its stability than the RNases extracted from roots, stem and cotyledons.



Our results suggest that the enzymes extracted from different tissues of Pitiúba cowpea seedlings are ribonucleases and not nucleases. Even though there are great similarities among the different RNases studied, the enzyme from leaves seems to be more stable to heat and to chemical denaturation, than roots and stem RNases.



We thank Daniel J. Rigden for reviewing the English. This work was supported by Financiadora de Estudos e Projetos (FINEP) and by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).



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1.  Received:  22/2/2001   - Accepted:  15/9/2001
2. Biólogo, Bolsista CNPq
3. Eng. de Alimentos, Bolsista CNPq
4. Biólogo, M.S., Dr.,  Prof. Adjunto IV, Pesquisador CNPq
5. Eng. Agron., M.S., Ph.D., Prof. Emérito, Pesquisador CNPq
6. Eng. Quim., M.S., Dr., Prof. Adjunto IV, Pesquisador CNPq - Corresponding author: Fax: 0XX-85-288-9829; e-mail:

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