Rat supernatant protein factor-like protein (SPF2) shares 90% sequence identity with rat SPF and 77% identity with human SPF, both of which have been shown to stimulate squalene monooxygenase in the cholesterol biosynthetic pathway. SPF2 appears to be predominantly expressed in respiratory and epithelial tissues, whereas SPF is expressed in liver. To determine if SPF2 was also able to stimulate squalene monooxygenase activity, we have cloned, expressed, and purified the protein following heterologous expression in Escherichia coli. SPF2 was only half as effective as SPF in stimulating squalene epoxidation and was more strongly inhibited by GTP and GDP. The inhibition by guanine nucleotides was fully prevented by alpha-tocopherol, a reported ligand for these proteins. Incubation of SPF2 with protein kinase A and ATP increased its activity by about twofold, has been found for SPF. These results indicate that SPF2 activity is modulated by guanine nucleotides and alpha-tocopherol, as well as by phosphorylation. (C) 2004 Elsevier Inc. All rights reserved.