Folate Utilization by Monomeric versus Heterotetrameric Sarcosine Oxidases

There are two types of bacterial sarcosine oxidases. The heterotetrameric enzymes contain subunits ranging in size from about 10 to 100 kDa, noncovalently bound FAD and NAD+, and covalently bound FMN attached to the β subunit (42–45 kDa). Monomeric sarcosine oxidases are similar in size to the β subunit in the heterotetramers and contain covalently bound FAD. Formaldehyde formation during sarcosine oxidation by several heterotetrameric sarcosine oxidases was suppressed in the presence of 50 μm[6S]-tetrahydrofolate, accompanied by a 25–50% increase in the rate of sarcosine oxidation. In contrast, [6S]-tetrahydrofolate caused only a modest decrease in the rate of formaldehyde production with monomeric sarcosine oxidases (∼25%), an effect which was virtually entirely attributable to an accompanying decrease in the rate of sarcosine oxidation. In the presence of 100 μm[6R,S]-tetrahydropteroyltriglutamate [H4Pte(Glu)3], the heterotetrameric enzymes catalyzed the formation of 5,10-methylenetetrahydropteroyltriglutamate [5,10-CH2-H4Pte(Glu)3] at a rate which was 35–60% faster than the rate of sarcosine oxidation in the absence of folate. An apparentKmvalue of 3.1 μmwas estimated for [6S]-H4Pte(Glu)3with the heterotetrameric corynebacterial sarcosine oxidase. In contrast, slow formation of 5,10-CH2-H4Pte(glu)3was detected during sarcosine oxidation with monomeric sarcosine oxidases, attributable to the nonenzymatic reaction of free formaldehyde with H4Pte(Glu)3. The results show that only the heterotetrameric sarcosine oxidases can use tetrahydrofolates as substrates and, in this regard, they resemble mammalian sarcosine and dimethylglycine dehydrogenases.