Enzymatic degradation of rhodotorula gracilis yeast cell wall

Abstract

The objective of present study was to assess the possibility of introducing enzymatic technique for the milder recovery of intracellular lipids from $\mbox{\underline{R}}$. $\mbox{\underline{gracilis}}$. A F5 fungus producing cell wall lytic enzyme was isolated from soil. This lytic F5 fungus was identified as the species of $\mbox{\underline{Aspergillus}}$ $\mbox{\underline{fumigatus}}$ group. Crude lytic enzyme of F5 fungus was of inducible exo-enzyme, and it was able to promote the lysis of $\mbox{\underline{R}}$. $\mbox{\underline{gracilis}}$ intact cells in fattening phase and growing phase. $\mbox{\underline{R}}$. $\mbox{\underline{gracilis}}$ intact cells in growing phase were more susceptible to lysis than those in fattening phase. Lytic enzyme system was composed of, at least, lytic enzyme and protease, which acted cooperatively in the lysis of intact cells. Lytic enzyme and protease were separated on Bio-Gel P-60 column chromatography. Lytic enzyme was not able to hydrolyze $\beta$-1, 3- and $\beta$-1,6-glucan which were known to the main types of bonds found in general yeast cell wall, such as $\mbox{\underline{Ascomycetous}}$ $\mbox{\underline{yeast}}$. The specificity of lytic enzyme remained obscure. This lytic enzyme alone was sufficient to hydrolyze the fractionated cell wall of $\mbox{\underline{R}}$. $\mbox{\underline{gracilis}}$ (alkali-insoluble-residues). This result well supported the hypothetical structure of yeast cell wall, which suggested the presence of mannoprotein complexes on the outer layer of yeast glucan. Lytic enzyme had its pH optimum between pH 4-4.5 and had its temperature optimum 50$^\circ$C. And, it had broad pH stability between 3.5 and 7.5. Heating of this enzyme for 20 min at 50$^\circ$C resulted in 80\% loss in enzyme activity. When the protease was assayed with azocasein, its pH optimum and temperature optimum were pH 8.0-9.5, 50$^\circ$C, respectively.