FOLATE UTILIZATION BY MONOMERIC VERSUS HETEROTETRAMERIC SARCOSINE OXIDASES

There are two types of bacterial sarcosine oxidases. The heterotetrame ric enzymes contain subunits ranging in size from about 10 to 100 kDa, noncovalently bound FAD and NAD(+), and covalently bound FMN attached to the beta subunit (42-45 kDa). Monomeric sarcosine oxidases are sim ilar in size to the beta subunit in the heterotetramers and contain co valently bound FAD. Formaldehyde formation during sarcosine oxidation by several heterotetrameric sarcosine oxidases was suppressed in the p resence of 50 mu M [6S]-tetrahydrofolate, accompanied by a 25-50% incr ease in the rate of sarcosine oxidation. In contrast, [6S]-tetrahydrof olate caused only a modest decrease in the rate of formaldehyde produc tion with monomeric sarcosine oxidases (similar to 25%), an effect whi ch was virtually entirely attributable to an accompanying decrease in the rate of sarcosine oxidation. In the presence of 100 mu M [6R,S]-te trahydropteroyltriglutamate [H(4)Pte(Glu)(3)], the heterotetrameric en zymes catalyzed the formation of 5,10-methylenetetrahydropteroyltriglu tamate [5,10-CH2-H(4)Pte(Glu)(3)] at a rate which was 35-60% faster th an the rate of sarcosine oxidation in the absence of folate. An appare nt K-m value of 3.1 mu M was estimated for [6S]-H(4)Pte(Glu)(3) with t he heterotetrameric corynebacterial sarcosine oxidase. In contrast, sl ow formation of 5,10-CH2-H(4)Pte(glu)(3) was detected during sarcosine oxidation with monomeric sarcosine oxidases, attributable to the none nzymatic reaction of free formaldehyde with H(4)Pte(Glu)(3). The resul ts show that only the heterotetrameric sarcosine oxidases can use tetr ahydrofolates as substrates and, in this regard, they resemble mammali an sarcosine and dimethylglycine dehydrogenases. (C) 1997 Academic Pre ss.