Matriz de evaluación

and α-Glucosidase Inhibitory Activities of Selected

Lactic Acid Bacteria

A version of this chapter has been published. Ramchandran, L., and Shah, N. P. 2008. Proteolytic profiles and angiotensin-I converting enzyme and α-glucosidase inhibitory activities of selected lactic acid bacteria. Journal of Food Science. 73 (2): M75-M81

3.1 Introduction

Lactic acid bacteria (LAB) are the most common microorganisms found in dairy products and therefore are one of the most extensively studied groups of microorganisms, of

which Lactobacillus, Lactococcus, Streptococcus and Bifidobacterium genera are most

common (Christensen et al., 1999). However, since the discovery of probiosis by Metchnikoff, substantial scientific interest has been directed towards the organisms that provide health benefit. The term ‘probiotic’, meaning ‘for life’, originated to describe substances produced by one microorganism which stimulate the growth of others. Among

the LAB, Lactobacillus and Bifidobacterium are the genera considered as probiotics that

have a considerable safety record and have been widely exploited by the manufacturers, resulting in a whole range of probiotic fermented milks (Stanton et al., 2003a; Shah 2007).

In the process of milk fermentation, the proteolytic abilities of LAB used as starters plays an important role because it provides these organisms the amino acids essential for their growth (Savijoki et al., 2006). The conversion of peptides to free amino acids and the subsequent utilization of these amino acids is therefore, a central metabolic activity in LAB (Christensen et al., 1999). The utilization of caseins is initiated by cell-envelope proteinase (CEP) which is critical for the growth of LAB in milk because they hydrolyse caseins into

more than 100 smaller peptide fragments. The 2nd

The proteolytic capabilities not only influence the flavour and textural characteristics of the products that contain these LAB, but also generate a host of peptides that are increasingly being associated with health beneficial properties, the bioactive peptides, which has renewed the scientific interest in these capabilities. Proteolysis by naturally occurring step in casein utilization includes transportation of peptides generated by CEP into the cell by the action of multiple oligopeptides transport (Opp) systems. After the casein-derived peptides are taken up by the cells, they are degraded by the action of peptidases with differing and partly overlapping specificities (Kunji et al., 1996). Collectively, these enzymes can remove the N-terminal amino acids from a peptide, the specificity depending on the peptide length and the nature of the N-terminal amino acid residue (Kunji et al., 1996; Christensen et al., 1999). Several researchers have observed some differences between the proteolytic systems of LAB (Fernandez-Espla and Rul, 1999; Morel et al., 1999; Lamarque et al., 2001; Letort et al., 2002; Schell et al., 2002; Altermann et al., 2005)

enzymes in milk or by microbial enzymes, especially from LAB, generates bioactive peptides during milk fermentation, thereby enriching the fermented product. Bioactive peptides have been defined as ‘specific protein fragments that have a positive impact on body conditions or functions and may ultimately influence health’ (Kitts and Weiler, 2003). Once produced, these bioactive peptides may act in the body as regulatory compounds with a hormone-like activity (Gobbetti et al., 2002). Among the bioactive peptides derived from milk proteins, those with blood pressure lowering effect are receiving special attention due to the widespread prevalence of hypertension. Blood pressure regulation is partially dependent on the rennin-angiotensin system. The angiotensin-I converting enzyme (ACE) regulates peripheral blood pressure and its inhibition can exert antihypertensive effects. The technological challenge therefore lies in the manufacture of fermented dairy products with a high concentration of these bioactive peptides. The single most effective way to increase the number of bioactive peptides is to ferment or co-ferment with highly proteolytic strains of LAB (Meisel et al., 1997). However, due to the inherent variations in these capabilities, selection of microorganisms to be used in fermented products, particularly those with specific health claims, is gaining importance. So far a comprehensive study on the enzyme profile, proteolytic activity as well as the ACE-inhibitory activities of yoghurt starter and

probiotic organisms has not been conducted. No work has been reported on the α-

glucosidase inhibitory activity of LAB.

The current study examines the proteolytic profiles and the angiotensin-I converting

enzyme (ACE) and α-glucosidase (α-glu) inhibitory activities of selected strains of yogurt

starter and probiotic organisms.

3.2 Materials and Methods

3.2.1 Bacterial strains and their activation

The pure strains of Streptococcus thermophilus 1275 and 285, Lactobacillus casei

2607 and 15286 and L. acidophilus 4461 and 33200 were acquired from the culture

collection of Victoria University and L. delbrueckii ssp. bulgaricus 1092 and 1368 and

Bifidobacterium longum 5022 were obtained from Australian Starter Culture Research Centre Ltd (Werribee, Vic, Australia). All the strains were activated from their frozen forms

(stored in 40% glycerol at -80 °C) by giving one transfer in MRS broth, except

S. thermophilus which was first transferred in M17 broth. This was followed by 2 successive transfers into 12% sterile reconstituted skim milk (RSM) supplemented with 2% glucose and

1% yeast extract, followed by 2 transfers into sterile RSM. During activation, all organisms

were incubated at the optimum temperature of 37 °C, except for L. delbrueckii ssp.

bulgaricus which was incubated at its optimum temperature of 42 °C, for 20 h. The 2nd

The selection of cultures for further studies was based on the results of this study, particularly the ACE-inhibitory activity. Since these cultures were to be used for yogurt making, the incubation period for which is usually 4 to 5 h, the incubation period selected for this study was 6 h. Although this period of incubation would be considered as short for

organisms such as L. acidphilus, L. casei and B. longum, however, since these organisms

were studied along with the faster growing yogurt cultures, it was necessary to maintain a uniform period of incubation for all organisms included in this study.

RSM transfer was used for the conducting the experiments at 37 °C. The rate of inoculation was 1% (v/v) and samples for the various analyses were removed after 0, 3 and 6 h of incubation.

3.2.2 Growth and pH

The growth and changes in pH were monitored as indicators of growth pattern after 0, 3 and 6 h incubation at 37 °C for all the organisms grown in RSM. The pH was measured using a pH meter (model 8417, Hanna Instruments, Singapore). Since each culture was taken individually for this study, the colony counts were determined using MRS agar for all

cultures except S. thermophilus, for which M17 agar was used (Shah, 2000a). The plates

were prepared by pour plate technique and incubated in anaerobic jars (with the exception of

S. thermophilus, which was incubated aerobically) at their optimum temperatures, as mentioned above, for 72 h.

3.2.3 Preparation of trichloroacetic acid filtrate

The trichloroacetic acid (TCA) filtrates of samples, removed at 0, 3 and 6 h of incubation, was prepared by mixing 5 mL sample with 1 mL MilliQ water and 10 mL of 0.75 N TCA followed by centrifugation at 4000 × g for 30 min at 4°C. The supernatant thus obtained was passed through a 0.45 µm membrane filter and stored at -20 °C until assayed.

3.2.4 Determination of extent of proteolysis

Proteolytic activity of all cultures was determined by the method of Church et al.

(1983). Three milliliters of OPA reagent was added to 150 µL of the TCA filtrate and vortexed for 5 s. Absorbance at 340 nm was measured after 2 min at room temperature,

3.2.5 Enzyme assays

The aminopeptidase and dipeptidase activities of the extracellular (EE) and intracellular extracts (IE) of all the organisms were determined. The extracts of the

organisms grown in MRS broth, except for S. thermophilus which was grown in M17 broth,

were prepared according to the method of Shihata and Shah (2000). Briefly, after incubation for 18 h, the cells were collected from the growth medium by centrifugation at 12000 × g at 4 ºC for 15 min while the supernatant obtained was used as the EE for enzyme assays. The pellet obtained was washed twice using 0.9% NaCl solution and centrifuged each time to remove the NaCl solution. The washed pellet was dispersed in 1 mL 0.05 M Tris-HCl buffer, pH 8.5, and sonicated for 5 min at 4 ºC. The suspension was then centrifuged at 12000 × g at 4 ºC for 15 min. The supernatant thus obtained was used as the IE for the enzymatic assays. The protein contents of EE and IE were determined as per the method of Bradford (1976).

The aminopeptidase activity of the extracts of all the organisms was determined by

the method of Fernandez-Espla et al. (1997) using chromogenic substrates (para-

nitroanilide-derivatives of L-isomers of Leu, Pro, Lys, Arg, Ala and Met). The assay mixture consisted of 100 µL of the IE, 400 µL of 50 mM Tris HCl buffer (pH 7.0) and 50 µL of 10 mM of each substrate. The mixture was incubated at 37 ºC for 20 min, after which the reaction was terminated by the addition of 1 mL acetic acid (30%, w/v). The concentration

of p-nitroanilide released was determined by measuring the absorbance at 410 nm using

NovaSpec®-II UV-Spectrophotometer. The enzyme activity was calculated considering

molar absorption coefficient of p-nitroanilide as 9024 mol-1cm-1

The dipeptidase activity of the extracts of all organisms was determined by the method of Wohlrab and Bockelmann (1992) using Ala-Met, Leu-Tyr, Leu-Gly, Ala-His and Pro-Ile as substrates. The reaction mixture contained 10 µL of the IE, 415 µL of 50 mM Tris-HCl buffer (pH 7.5), 50 µL of 22 mM of each substrate solution, 25 µL of peroxidase solution (5 mg/mL of 0.8 M ammonium sulphate solution), 25 µL of L-amino oxidase

solution (2 mg/mL deionised water) and 25 µL of 11.5 mM o-dianisidine solution. The

mixture was incubated at 50 ºC for 20 min followed by the addition of 50 µL of 120 mM dithiothreitol solution to terminate the reaction. The enzyme activity was calculated using a molar absorption coefficient of 8100 mol

and expressed as micromoles of substrate hydrolysed per millilitre of extract per min. The specific activity was calculated and expressed per milligram of protein.

-1

cm-1and expressed as micromoles of substrate

hydrolysed per millilitre of extract per minute. The specific activity was expressed per milligram of protein.

3.2.6 ACE-inhibitory activity

The ACE-inhibitory activity of the TCA filtrates of all organisms after 3 and 6 h growth at 37 °C was determined by the method of Cushman and Cheung (1971) and Donkor et al. (2005) with some modifications. The method involved the use of 5 mM Hip-His-Leu in 0.1 M borate buffer containing 0.3 M NaCl, pH 8.3, as substrate and rabbit lung ACE (0.1 units/ml). To 200 µL of the substrate solution, 60 µL of the borate buffer and 30 µL of the TCA filtrate were added and pre-incubated at 37 °C for 5 min. This was followed by the addition of 20 µL of the enzyme solution and incubation at 37 °C for 30 min. The reaction was terminated by adding 250 µL of 1 M HCl solution and then mixed with 1.7 mL ethyl acetate. After 10 min of quiescent standing, 1.4 mL of the ethyl acetate layer was siphoned out and dried on a boiling water bath and then in an oven maintained at 80 °C for 30 min. The residual hippuric acid was dissolved in 1 mL of deionized water and absorbance measured at 228 nm using UV/Vis spectrophotometer (Pharmacia, LKB-UltrospecIII). The per cent inhibition was calculated using the following formula:

ACE-inhibition (%) =_1 _×100 − − − B A D C where

A is the absorbance in the presence of ACE and without sample, B is the absorbance without ACE and sample, C is the absorbance with ACE and sample and D is the absorbance with

sample but without ACE. The ACE-inhibition was also expressed in terms of IC50, defined

as the protein concentration in sample (mg/mL) required to inhibit 50% of the ACE activity. The protein in the TCA filtrates was determined by the method of Lowry et al. (1951) using bovine serum albumin as standard.

3.2.7 α-Glucosidase inhibitory activity

The method of Zhang et al. (2007) was adopted with some modifications to measure

the α-glu inhibitory activity of the TCA filtrates of cultures after 3 and 6 h of growth at

37 °C. To 300 µL of 0.1 M phosphate buffer (pH 6.5) 150 µL of 20 mM p-nitrophenyl-α-

glucoside (SIGMA Chemicals, St Louis, MO, USA) solution and 50 µL of the sample were

added and the mixture pre-incubated at 45 °C for 10 min. To this, 100 µL of α-glucosidase

enzyme solution (0.2 units/mL, from yeast, SIGMA Chemicals) was added and incubated for another 10 min at 45 °C. The reaction was terminated by adding 2 mL of 0.1 M sodium carbonate solution and the amount of p-nitrophenol released was determined by measuring

the absorbance at 400 nm using NovaSpec®-II UV-Spectrophotometer. The percent

3.2.8 Statistical analysis

Data were analysed using 1-way analysis of variance at 95% level of significance (Albright et al., 1999). All experiments were replicated twice and all analyses were carried out in triplicates. The results presented are a mean of 6 observations ± standard deviation (SD).

3.3 Results and Discussion

3.3.1 Growth and pH

Changes in growth and pH of all the organisms at 3 hourly intervals during growth in RSM at 37 °C are shown in Figures 3.1 and 3.2, respectively. The variations in initial numbers (0 h) of the different types of organisms is a consequence of the variations in growing ability of these organisms which is also strain specific. All organisms showed increase in log counts after 3 and 6 h of growth (Figure 3.1) with corresponding decrease in

pH (Figure 3.2). Among the yogurt cultures, both the strains of S. thermophilus showed a

similar trend of growth which reflected in a parallel trend of decreasing pH. On the other hand, strain variations in the growth and therefore the decrease in pH was observed for the

two strains of L. delbrueckii ssp. bulgaricus. The growth of L. delbrueckii ssp. bulgaricus

1368 was slower during the initial 3 hours but improved in the subsequent 3 h of growth, being significantly (P < 0.05) higher than strain 1092. A similar pattern was also observed in

the decrease in pH of RSM for these 2 strains of L. delbrueckii ssp. bulgaricus. Donkor et al.

(2007a) have also reported slower growth rate for strains of L. delbrueckii ssp. bulgaricus

than that for S. thermophilus. Chandan and O’Rell (2006) have indicated that L. delbrueckii

ssp. bulgaricus has poorer ability to breakdown peptides to free amino acids than

S. thermophilus. This indicates that L. delbrueckii ssp. bulgaricus are more fastidious than

S. thermophilus and need simpler sources of nitrogen for their growth. Hence, when grown

individually, L. delbrueckii ssp. bulgaricus takes a longer time to establish in milk as a

growing medium that contains complex proteins as a source of nitrogen. Strains of probiotic

organisms showed lower growth rates than S. thermophilus and therefore less decrease in pH

of the medium. Both the strains of L. casei, L. acidophilus 33200 as well as B. longum 5022

showed very little growth during the first 3 h of incubation but showed significantly (P < 0.05) improved growth in the subsequent 3 h of incubation. In contrast, the drop in pH was significant (P < 0.05) throughout the 6 h of incubation for all the probiotic organisms.

3.3.2 Proteolytic activity

Lactic acid bacteria are fastidious organisms that require an external source of amino acids or peptides that are provided by the most abundant and proline-rich milk proteins, caseins (Savijoki et al., 2006). Thus, proteolytic capabilities of these organisms are important to ensure their growth in milk. The ability of the organisms to break down the milk proteins was determined by measuring the free amino acids generated after 0, 3 and 6 h of growth at 37 °C and is presented in Figure 3.3. All the organisms showed progressive increase in the free amino acids content with time, although the increase for all the organisms was not significant (P < 0.05) during the first three hours after which all organisms showed a significant (P < 0.05) improvement in their proteolytic capabilities. Donkor et al. (2007a) have also reported variations in the extent of proteolysis by strains of LAB with time, although they observed significant differences only after 12 h of growth. However, Leclerc et al. (2002) have reported a linear increase in the extent of proteolysis with fermentation

time for L. helveticus. Among the species of LAB studied, B. longum surprisingly showed

very high proteolytic capability. The ability of B. longum to utilise almost all types of

substrates (as shown in Tables 3.1 and 3.2) could explain their high proteolytic capability. It is generally accepted that bifidobacteria have poor ability to grow in milk due to their poor

proteolytic activity (Dave and Shah, 1998a). Although differences among species of LAB

were observed in their ability of utilise milk proteins, no significant (P < 0.05) differences were observed in the proteolytic capability among the different strains of any of the

organisms, with the exception of those of S. thermophilus. These patterns of proteolysis

corresponded with the growth patterns of these organisms. Several researchers have reported wide variations in the proteolytic abilities of LAB (Hickey et al., 1983; Oberg et al., 1991).

3.3.3 Aminopeptidase and dipeptidase activity

The specific activity of aminopeptidases and dipeptidases in the extracellular and intracellular extracts of all the organisms is shown in Table 3.1 and 3.2. Variations in the specific activity of aminopeptidases (Table 3.1) were observed for the various species and strains of organism used and also for the various substrates. These differences were significant (P < 0.05) between strains of the same species of organism and also between the different species. Different species showed different affinity for the substrates tested, some

of which were significantly (P < 0.05) different. Both strains of L. casei and showed highest

aminopeptidase activity (sum of IE and EE) to all substrates except lysine while both strains of S. thermophilus showed for lysine. Lactobacillus acidophilus 4461 showed highest

activity to proline and alanine containing substrates. Among all the organisms, only the EE of S. thermophilus 1275 and the IE of L. acidophilus 4461 and L. delbrueckii ssp. bulgaricus

1092 were able to utilise all the substrates. Similarly, among all the substrates used, only the

para-nitroanilide derivates of proline, alanine and arginine were hydrolysed by the EE of all

the organisms except those of L. casei, while the derivatives of proline and lysine were

hydrolysed by the IE of all the organisms. Although it is generally accepted that most aminopeptidases and dipeptidases are located intracellularly, presence of some peptidases in

the cell wall fraction of L. delbrueckii ssp. bulgaricus (Gilbert et al., 1994) and presence of

extracellular peptidases in the proteolytic pathway of LAB have also been implied (Kunji et al., 1996). Some exceptions were also observed during this study where the EE of some of the organisms showed higher aminopeptidase activity than the IE for some of the substrates

used. Both the strains of S. thermophilus showed higher EE activity to derivatives of proline,

alanine and methionine, while both strains of L. casei showed higher EE activity to proline

containing substrate only. On the other hand L. delbrueckii ssp. bulgaricus 1368 showed

higher EE activity to alanine, proline and arginine containing substrates while B. longum

5022 showed higher EE activity to proline and methionine containing substrates. Shihata and

Shah (2000) have also found higher EE activities for aminopeptidase activity of strains of B.

longum and L. acidophilus to arginine.

On the other hand, dipeptidases of both, the EE and IE of all the organisms (Table 3.2), exhibited broad substrate specificity and were able to utilise all the five substrates to

varying extents, with the exception of EE of both strains of S. thermophilus. Among the

species examined, L. delbrueckii ssp. bulgaricus showed better dipeptidase activity for the